Added Bitbucket Server regex for urls.

[chris5287]

This adds support for Bitbucket server urls, which are different to Bitbucket cloud urls (see #180)

Example code running: https://dotnetfiddle.net/gZWblj

[chris5287]

@ctaggart ping it would be great to get these changes in the next release please!

[ctaggart]

Thanks for this contribution! This depends on the unit testing #250 being merged first. This will need some unit tests. As is right now, this will probably break SourceLink.Create.CommandLine:

https://github.com/ctaggart/SourceLink/blob/master/SourceLink.Create.CommandLine/CreateTask.cs#L68-L78

This code should probably go in a:
SourceLink.Create.BitbucketServer.UrlConverter

The original intention was that we would create a nupkg of:
SourceLink.Create.BitbucketServer

But then we decided to add multiple implementations to SourceLink.Create.CommandLine. Were you wanting to use this new UrlConverter with the CommandLine version or the Libgit2 based creates?

fyi, I'm on vacation for a week starting now, so I'll respond when I get back.