Add hint target 'run' to execute a command based on a hint URL.

Fixes Issue #285.
The command is not preceded by a colon because there is no need to
distinguish between a search and a command.
Hints: Use the src html attribute in addition to href in _resolve_url.

See issue #286
Merge branch 'master' of https://github.com/krobelus/qutebrowser into krobelus-master
Cleanups in hint.py.
Merge branch 'krobelus-master'
Update docs