Add a way to back up notes (via git)

`$ note push` or something similar to push notes to a remote git repo.

EG ->
```
gitPushDate=$(date +%Y-%m-%d)
echo "$gitPushDate"
git add ~/notes/*
git commit -m 'Automatically adding notes through $gitPushDate'
git push origin master
```
* Requires a git repo be set up and `origin` configured.