Add URI escaping for hashtags.

Hashtags are now able to contain characters which aren't valid as query
parameter because these are now escaped.

Fixes #157.
Use hiccup for linkifying.

Using hiccup deals with many URL related stuff like escaping which
should be safer than assembling links by ourself.
Merge pull request #158 from mvitz/157-fix-hashtag-with-caret

Fix #157: caret in hashtag causes 500 error