Ability to run a single task by task name

Fixes #30
Allow users to append 'Huxleyfile.json' to globs by themselves
Display task name during recording
Change API from callback to promise
Clean up code a bit
Fix screenshot update not creating dir bug
Move out example into a dedicated repo
Implement default workflow; public API change

API now uses promises rather than callbacks.
Default workflow is basically:

- git stash
- write screenshots
- git unstash
- compare screenshots
- keep different ones
Add tests to loadRunnables