add contextual menu commands

[petty]

this allows for the "right-click" contextual menu to show shortcuts for viewing the current file

[Short description of suggestion]

Steps which explain the enhancement

1. view any file (that is on github)
2. select the file, a line, or lines like you normally would with "Open On Github"
3. Leave your cursor right where it is & right click with your mouse or trackpad

Current and suggested behavior

this adds easier & maybe faster access to Open On Github functionality

Why would the enhancement be useful to most users

This adds another way to access existing functionality

Atom Version: 1.10.2 OS and Version: OS X 10.11.6

[50Wliu]

@danielbayley That wouldn't work since Open on GitHub looks at the active pane item to determine which repo is open. Having the commands on the Tree View would mean that the Tree View would be the active pane item and so nothing would happen.

[UziTech]

@50Wliu this would work once #111 is merged