Shortcodes: Medium: Improve URL path type matching

[aduth]

Currently, user stories are interpreted as profiles, as they begin with an "@"-prefixed path. The patterns introduced in this pull request improve matching against profiles and collections, including more accurate supported characters.

Example URLs to test against:

• Profile: https://medium.com/@ev
• Collection: https://medium.com/the-story
• User Story: https://medium.com/@ev/why-i-m-glad-jack-is-back-at-twitter-9bbda2ae5dca#.wccthwtfl
• Collection Story: https://medium.com/the-story/taking-medium-to-the-next-level-cb7f223fad86#.3c3u7xrgk

Supported characters are not widely published, but were inferred as follows:

• For user names, numbers, letters, dots, and underscores are permitted. You can verify this by attempting to change your username from the Medium settings page to include any character not in this set, and observe the error message.
• For collections/publications, supported characters were inferred by attempting to include a variety of special characters and letters in the Medium New Publication form. After creating the Publication, all non-letters/numbers are converted to dashes.

[zinigor]

Looks good, works as expected, thank you!