Added git-sqwish(1)

Included in this pull request:

- Added `git-sqwish(1)` which compresses changes from **current branch** into one commit.
- Manual files and README updates for `git-sqwish(1)`

I use a variant of `git-sqwish(1)` every day at work since I work on a pull request system which demands all branches are ready to merge once reviewed.

I cannot use `git-squash(1)` since that requires being on `master` and merging from there. Everything we do is done via the GitHub UI.