Git Repo Search On Linux for when vim instance is inside a Git Repo

Hi @arecarn, great tool!  I stumbled upon it thanks to your website.  Anyways, I wanted to contibute to the tool by automatically searching github (against the repo you are in), however, I have not yet coded for exceptions (when you are not in a repo for example).  I'm not a strong vimscripter but I tried to stay within the flow of your code.

This also depends on the use of `git`, `grep`, and `cut` utilities, but I would expect that these are common defaults for Linux users.
`let repo = system('git remote show -n origin | grep Fetch | cut -d: -f3') `

I had a weird blank screen issue that is explained by the following:
http://stackoverflow.com/questions/18616873/vim-blank-screen-on-subsequent-shell-commands
This may or may not be a problem for Windows and Mac OSX, but executing the command in a scratch buffer worked out my issues.

Another issue is that forked repos cannot be searched (not indexed by Github, unless more popular than original repo), so you'd have to leverage using Github API or other 3rd party tool to access the parent repo.

Anyways, I think you have an excellent tool, and I'm happy to read your feedback.