Add new branch option and context option to open githubinator on master

Hello folks!

I've made this because of a personal need, sometimes I want to see/send the URL pointing to master instead of my current checkout'd branch.

This could obviously be a new `sublime-setting`, but I'm not sure if this is really needed, looking forward for your comments and feedbacks, even because these are my first python lines.

Also this adds a valid option for the use case described here https://github.com/ehamiter/ST2-GitHubinator/issues/24

Thanks for your attention and for the great plugin!