Fix GitHub directive to support line numbers

[jonas]

Reported in akka/akka-http#1416 (and akka/akka-http#900).

[jonas]

I managed to pull off the solution mentioned here akka/akka-http#1416 (comment) so it is possible to link to a snippet label and ensure the file exists and that line numbers don't get out of date.

@github[example](/src/test/scala/ServerSetup.scala) { #ssl-setup }

[jonas]

One nice to have for the GitHub tree paths, is to check if the path exists iff in the GitHub tree URL (i.e. /tree/$version) matches project.version or points to master. Does this make sense?

[pvlugter]

Nice!