Use strip_prefix to please clippy

This fixes the following clippy warning:

error: stripping a prefix manually
   --> src/cursor/find_cursor.rs:266:36
    |
266 |                     let mut path = &path[1..];
    |                                    ^^^^^^^^^^
    |
    = note: `-D clippy::manual-strip` implied by `-D warnings`
note: the prefix was tested here
   --> src/cursor/find_cursor.rs:264:17
    |
264 |                 if path.starts_with('~') {
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip
help: try using the `strip_prefix` method
    |
264 ~                 if let Some(<stripped>) = path.strip_prefix('~') {
265 |                     theme_dir.push(&home);
266 ~                     let mut path = <stripped>;
    |

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/cursor/find_cursor.rs

@@ -258,9 +258,8 @@ where
                 // Calculate the path to the theme's directory
                 let mut theme_dir = PathBuf::new();
                 // Does the path begin with '~'?
-                if path.starts_with('~') {
+                if let Some(mut path) = path.strip_prefix('~') {
                     theme_dir.push(&home);
-                    let mut path = &path[1..];
                     // Skip a path separator if there is one
                     if path.chars().next().map(std::path::is_separator) == Some(true) {
                         path = &path[1..];