add flag to ignore dirty local git clone

[mmguero]

When doing zkg install, a check is made to ensure the local repository isn't dirty. If it is, the install fails.

For development purposes, it would be convenient to have a flag named something like --ignore-dirty-git that would allow the install operation to proceed anyway.

e.g.

$ touch asdfasdf

$ zkg install .
error: local git clone at . is dirty

$ zkg install --ignore-dirty-git .
The following packages will be INSTALLED:
...

related to #145

[awelzel]

Reopening as the change was reverted with #179

[mmguero]

Thanks, I had noticed this behavior ("The existing implementation isn't function and behaves confusingly (silently discards the actual changes during install/test)") and hadn't gotten around to trying to figure out if I was doing something wrong or if it was a problem with zkg itself.