Added support for symbol overrides. Fixes #41

As discussed in #41, we have added the ability to override status symbols since not all fonts support our preferred set. I have chosen to leave the suggestions out of this PR as it is starting to get quite bulky. In this PR:

- Moved to predictable naming scheme for status cases (e.g. `(dirty-?)?(unpushed-?)?(unpulled)?`)
- Added tests for using symbol overrides
- Implemented symbol overrides
- Updated screenshot to match naming scheme
- Added documentation and screenshot for symbol overrides

/cc @rpdelaney