Allow slashes in path params

this has been brought up in previous (closed) issues such as #174 and #566.

there are some situations where this could be useful. an example would be a filesystem browser + editor, with routes that correspond to the file's path like so:

• GET /files/{path:*}/edit
• DELETE /files/{path:*}/delete
• POST /files/{path:*}/upload

path:* would match 0 or more path segments, meaning the following urls are valid:

• /files/one/two/three/four/edit
• /files/foo/bar.txt/edit
• /files/upload

i'm willing to open a PR to implement this.