Completions for URL part in routes

* Suggestion proposals are scoped to the document, i.e, only parts
   already existing in the edited route file are suggested.
* The proposals do not consider the controller method to provide
   better/smarter proposals (this could be a possible future improvement).
* There are some edge case where it's not clear what is the best set of
   proposals to offer to the user. For instance, when should the dynamic
   parts be suggested? After some thinking, I decided to only suggest
   dynamic parts if
   1) the users asks completion *before* entering the URI, or
   2) after a trailing slash.
   Let's see what's the reaction of the community to these defaults.
* Extracted common logic for testing HTTP method completion and URI completion.
* The icon show for URI proposals is currently the same as the on for HTTP
  method proposal (I've spent some time looking for a better one without luck).

Fix #82