Merge pull request #254 from spacewander/master

add welcome in git-repl.
Changelogs for version 2.0.0
Bump version in `bin/git-extras`

- Bump version to 2.0.0 in `bin/git-extras` to enable updating package
via `git-extras update`
Merge pull request #255 from sanusart/master

Bump version in `bin/git-extras`.
use mixin to add 'is git repo' check for each command