add git-wip and git-unwip

[acant]

• git-wip will commit all the changes in the working directory into a WIP commit
• git-unwip will undo a WIP commit using git-undo

I have been using these commands for a while, to save interim changes when switching between feature branches. This ideas in other places, but it would be nice to included it here too. I have added links to files in https://github.com/ddollar/git-utils which are similar, but the scripts are not exactly the same.

Any feedback is welcome. I went through the contributing guidelines, but if I have missed anything I am happy to updated it.

Thanks in advance.