Add tags option

**NOTE**: Don't merge this until you have merged #93 please (it *should* clear up the PR diff automatically once you've merged).

* Adds `add_tags` service property that will automatically add specified tasks.

Additional changes:
* Read configuration file in as `utf-8` bytes (to allow one to specify tags having unicode characters).
* Altered task diff comparison to be more robust.
* Cleaned-up private `_get_taskwarrior_record` method well enough to make it public.