Cannot specify URIs with a custom scheme in command line

[aperezdc]

After the fix for #4, using custom URI scheme handlers is not possible because Dinghy will try to prepend http:// to the URIs which have a custom scheme. For example the following doesn't work:

dinghy -d myfiles:/path/to/files myfiles:/

The command above will try to load http://myfiles:/, which of course will fail.