Add support for Source Folder external URL

[skyfex]

I know this feature pull request is probably incomplete. Creating it to get feedback

This pull request creates a feature where each Source Folder can have an external URL attached, which can be used to generate a link to the source code of a definition in the documentation. Example

Source Folder: docs/uvm-src
   Name: UVM 1.2
   Url: https://github.com/accellera/uvm/blob/UVM_1_2_RELEASE/distrib/src/{filePath}#L{lineNum}

• Can this feature be added?
• What should I fix/improve to complete it?

[NaturalDocs]

I haven't looked at the source but I like this idea a lot.

Accepting code contributions has always been tricky because I currently own all the code except SQLite, which lets me do certain things like offer versions under other licenses. I would have to write up some sort of contributor agreement to accept code from others while still maintaining that, which I haven't gotten around to because there haven't been many. My other option is to just implement the idea myself without looking at your code, which I think I'm leaning towards, in which case we can treat this as a feature request.

[skyfex]

It's fine by me to just donate the code and assign copyright to you. I'm in the process of double-checking with employer too.. Could you e-mail me at skyfex@gmail.com ?