Port gutter_(next|prev)_change commands from GitGutter

GitGutter has shortcuts to jump between changed parts of a file.

This commit resolves #5 by porting that functionality from GitGutter's
git_gutter_change.py file:
https://github.com/jisaacks/GitGutter/blob/master/git_gutter_change.py
Merge branch 'feature/pr19' into develop
Add notes on commands and key bindings for jumping between changes.
Merge branch 'feature/pr18' into develop
Update install, upgrade and readme messages.
Merge branch 'release/v1.1.0'