Source maps 🌐🌍

I switched lib to [maccman/sourcemap](/maccman/sourcemap) which is capable of concatenating them and is the one used by sprockets-4.

Main problem right now is —unsurprisingly— off-by-one errors, but in my brief manual tests were almost right :frog: 

See also: https://github.com/sstephenson/sprockets/issues/607#issuecomment-54520191