rename optimizer compiler flags

[lrytz]

remove the Ys in -Yopt, -Yopt-warnings, maybe not in -Yopt-inline-heuristics, -Yopt-trace

[lrytz]

also provide a new option extends the current l:project. provably a list of white-listed packages. a by-library option would be useful, but that's more disruptive (how to refer name a library on the command-line? how to know in the backend what library a class belongs to?)

[lrytz]

also check #31

[dwijnand]

Can be closed with scala/scala#5189 merged. (I was just in the neighbourhood)