Add GitHub provider

[diegobernardes]

These are the endpoints that will be contempled:

• https://github.com/owner/repository#anchor
• https://github.com/owner/repository/commit/ref#diff-ref
• https://github.com/owner/repository/blob/master/folder
• https://github.com/owner/repository/blob/master/folder/file
• https://github.com/owner/repository/blob/master/folder/file#L10-L15
• https://github.com/owner/repository/blob/master/folder/file#L10
• https://github.com/owner/repository/blob/ref/folder
• https://github.com/owner/repository/blob/ref/folder/file
• https://github.com/owner/repository/blob/ref/folder/file#L10-L15
• https://github.com/owner/repository/blob/ref/folder/file#L10
• https://github.com/owner/repository/tree/master/folder
• https://github.com/owner/repository/tree/master/folder/file
• https://github.com/owner/repository/pull/1/files#diff-refR16-R24

Done at #8

• https://github.com/owner
• https://github.com/owner/repository
• https://github.com/owner/repository/issues/1
• https://github.com/owner/repository/issues/1#issuecomment-2
• https://github.com/owner/repository/commit/ref
• https://github.com/owner/repository/pull/1
• https://github.com/owner/repository/pull/1/commits/ref

[diegobernardes]

@kabute, do you see any other useful pattern to match?