Removed generated files in servedocs

[tecosaur]

I've just tried using LiveServer.jl to work out a slightly tricky docs issue with a package of mine, and I've noticed that it leaves make.jl-generated files in the docs/src directory. I wonder if servedocs could use a try ... finally ... end block to record the src/ directory tree at the start, and immediately after running make.jl, and from the difference obtain a list of "generated files to remove" in the finally clause.

[tlienart]

Feel free to suggest a PR, as long as it's an extra keyword to servedocs which defaults to false (current behaviour) and as long as it doesn't add a dependency, I'm not against adding something like this in.