WIP: git-pretty-accept

@htanata Here's my work so far on git-pretty-accept. Why git-pretty-accept?

* pretty - because the branch is rebased first before merging. The local and remote PR branch are also automatically deleted.
* accept - because the PR branch is pushed to remote once merged.

To dos:

- [ ] Ask for merge message from user. 
- [ ] Display git calls while running.
- [ ] Display remaining uncalled git commands if any git command throws an error.

Let me know what you think!