Limit to github origin

[blois]

Should be able to limit the extension to only github URLs with:
https://developer.chrome.com/apps/match_patterns

[jakevdp]

I think it's useful for something to happen when the extension is clicked on a non-github page, otherwise users might think it just doesn't work.

[jakevdp]

Looking into this; if I'm understanding the chrome docs correctly, it looks like match patterns only apply to content scripts, which run with access to the content of the page. This extension uses a background script, so matching patterns do not apply.

[jakevdp]

I think also because this extension uses only the activeTab permission, match patterns are not applicable.