Add a .gitignore and avoid bad filenames on OSX

I have two trivial changes that fixed little issues for me:

1. ignore the plugin-generated files that relate to tags (tags.txt and index.pickle) to make install & upkeep of the plugin as a git repo hassle-free
2. strip colons from filenames converted from titles because they are forbidden in OSX filenames (it's some kind of dir separator for backwards-compatibility or some other nonsense).