Refactor and improve `git squash`

`git squash` refactor:
- first argument can be a branch or a commit reference
- the `--me` argument is not required anymore
- use `git reset --soft` is squashing on a current branch
- if squashing on master branch, prompt for confirmation
- does not delete provided branch after squashing
- fix bug: if `git merge --squash` command fails, exit script

Update README and man pages.

Fixes #95
Closes #198