Support blob links as part of Github Directive

[Manish3323]

There is an issue in sbt-paradox plugin when we set github.base_url having blob for versioned link, source_urls are not pointing correctly to branch/ specific.

As an example,
In our documentation, we are setting "github.base_url" -> "https://github.com/org/repo/*blob*/0.2.0".
we snip from /src/build.sbt file & then the source url of the corresponding build.sbt gets generated as https://github.com/org/repo/tree/master/src/build.sbt where it should have been https://github.com/org/repo/blob/0.2.0/src/build.sbt

This issue can be reproduced by going over this versioned link & clicking on Edit this page button at the bottom of the page. It points to master branch, where as it should be pointing to 4.0.1 tag. In a sense branch should be whatever we have set to github.base_url.

I think blob should be handled by the paradox plugin.

Possible solution would be updatingThis file to handle BlobUrl as well along with TreeUrl.

  lazy val TreeUrl = (s"(.*$githubDomain/[^/]+/[^/]+/tree/[^/]+)").r
  lazy val BlobUrl = (s"(.*$githubDomain/[^/]+/[^/]+/blob/[^/]+)").r

Could be like following