Useless sourcemaps created

[IvanSanchez]

gobble-replace creates a hi-res identity (1:1) sourcemap for all processed files, even if there have been no changes.

The problem with hi-res maps is that they are big, in some of my cases four times the size of the original file. This poses a performance problem for later steps in the gobble workflow (e.g. gobble-concat, where those maps are parsed).

Ideally, files without a matching replacement should not create a sourcemap.