feat(front): Change permalink variable names

General reasons
- The hope is this will make the meaning more clear
- This is more aligned with the planned document_attributes

Specific reasons
- path: unclear if includes filename or not (it doesn't)
- filename: unclear of includes extension or not (it doesn't)
- output_ext: overly verbose.  We don't call `path`, `output_path`, so
  why does this need it?

Finish fixing cobalt-org#198

BREAKING CHANGE: permalink variables renamed
- path -> parent
- filename -> name
- output_ext -> ext

`cobalt migrate` should take care of this.

src/cobalt_model/frontmatter.rs

@@ -13,7 +13,7 @@ use error::Result;
 use super::datetime;
 use super::slug;
 
-const PATH_ALIAS: &'static str = "/{{path}}/{{filename}}{{output_ext}}";
+const PATH_ALIAS: &'static str = "/{{parent}}/{{name}}{{ext}}";
 lazy_static!{
     static ref PERMALINK_ALIASES: HashMap<&'static str, &'static str> = [
         ("path", PATH_ALIAS),

src/document.rs

@@ -41,15 +41,13 @@ fn format_path_variable(source_file: &Path) -> String {
 fn permalink_attributes(front: &cobalt_model::Frontmatter, dest_file: &Path) -> liquid::Object {
     let mut attributes = liquid::Object::new();
 
-    attributes.insert("path".to_owned(),
+    attributes.insert("parent".to_owned(),
                       Value::Str(format_path_variable(dest_file)));
 
-    let filename = dest_file.file_stem().and_then(|s| s.to_str());
-    if let Some(filename) = filename {
-        attributes.insert("filename".to_owned(), Value::str(filename));
-    }
+    let filename = dest_file.file_stem().and_then(|s| s.to_str()).unwrap_or("");
+    attributes.insert("name".to_owned(), Value::str(filename));
 
-    attributes.insert("output_ext".to_owned(), Value::str(".html"));
+    attributes.insert("ext".to_owned(), Value::str(".html"));
 
     // TODO(epage): Add `collection` (the collection's slug), see #257
     // or `parent.slug`, see #323

src/jekyll_model/frontmatter.rs

@@ -75,6 +75,12 @@ fn migrate_variable(var: String) -> Part {
         let name: &str = &var;
         VARIABLES.contains(&name)
     };
+    let var = match var.as_str() {
+        "path" => "parent".to_owned(),
+        "filename" => "name".to_owned(),
+        "output_ext" => "ext".to_owned(),
+        x => x.to_owned(),
+    };
     let variable = if native_variable {
         format!("{{{{ {} }}}}", var)
     } else {
@@ -106,7 +112,7 @@ mod test {
     #[test]
     fn migrate_variable_known() {
         let fixture = "path".to_owned();
-        let expected = Part::Constant("{{ path }}".to_owned());
+        let expected = Part::Constant("{{ parent }}".to_owned());
         let actual = migrate_variable(fixture);
         assert_eq!(actual, expected);
     }
@@ -137,7 +143,7 @@ mod test {
     #[test]
     fn convert_permalink_known_variable() {
         assert_eq!(convert_permalink("hello/:path/world/:i_day/"),
-                   "/hello/{{ path }}/world/{{ i_day }}/".to_owned());
+                   "/hello/{{ parent }}/world/{{ i_day }}/".to_owned());
     }
 
     #[test]

src/legacy_model/frontmatter.rs

@@ -86,6 +86,12 @@ fn migrate_variable(var: String) -> Part {
         let name: &str = &var;
         VARIABLES.contains(&name)
     };
+    let var = match var.as_str() {
+        "path" => "parent".to_owned(),
+        "filename" => "name".to_owned(),
+        "output_ext" => "ext".to_owned(),
+        x => x.to_owned(),
+    };
     let variable = if native_variable {
         format!("{{{{ {} }}}}", var)
     } else {
@@ -109,7 +115,7 @@ mod tests {
     #[test]
     fn migrate_variable_known() {
         let fixture = "path".to_owned();
-        let expected = Part::Constant("{{ path }}".to_owned());
+        let expected = Part::Constant("{{ parent }}".to_owned());
         let actual = migrate_variable(fixture);
         assert_eq!(actual, expected);
     }
@@ -140,7 +146,7 @@ mod tests {
     #[test]
     fn convert_permalink_known_variable() {
         assert_eq!(convert_permalink("hello/:path/world/:i_day/"),
-                   "/hello/{{ path }}/world/{{ i_day }}/".to_owned());
+                   "/hello/{{ parent }}/world/{{ i_day }}/".to_owned());
     }
 
     #[test]