Add GitHubURLProviderInfo for converting GitHub URLs

[alexbainter]

Closes #131.

This branch adds a new property to ReferenceItem called url_provider_info which returns an instance of a new URLProviderInfo class. This class can be extended to provide provider-specific functionality for ReferenceItems based on their URL providers. In this case, GitHubURLInfo was added for GitHub urls, including raw_url and repo_url computed properties.

if reference_item.url_provider_info.provider_name == 'GitHub': # or GitHubURLInfo.provider_name
    print(reference_item.url_provider_info.raw_url)
    print(reference_item.url_provider_info.repo_url)

I considered just treating GitHub as a special case and adding these helpers directly to the ReferenceItem class but I wanted an approach that would work well with other providers down the road.

[lisad]

ooh a proper Class Factory!