Support other code hosts

[hadley]

e.g. GitHub enterprise (#676), GitLab (#1045), and sub-directories (#1152)

This requires at least the ability to configure the following URLs:

• A home page, equivalent to (e.g.) https://github.com/r-lib, for the navbar.
• A source url, equivalent to (e.g.) https://github.com/r-lib/pkgdown/blob/master/{path}, for reference and article source links.
• An issue url, equivalent to (e.g.) https://github.com/r-lib/issues/{number}, for NEWS links.
• A person url, equivalent to (e.g.) https://github.com/{username}, for NEWS links.

[hadley]

To do:

• Rename github_source_links() to source_links() and change first argument to full package object

• Rename add_github_links() to ??? and use issue and name urls. This will require switching to some more complex form of text replacement.

I think we could probably leave the navbar off since you're likely to want to change the icon too if you're not on GitHub.

[hadley]

Hmmm, currently GHE, GitLab, and subdirectories as have exactly the same form as github, so it might not be worth generalising too much at this point.