permit multiple source directories

[chrylis]

I'm compiling a LESS overlay for Bootstrap that just needs to refer to a few of the variables, like the grid size. I'm using Webjars to grab the Bootstrap source for the build, but currently I have to copy my own LESS source into a secondary directory, unpack Bootstrap there, and tell lesscss-maven-plugin to compile that into the final output directory. It would be helpful to be able to specify multiple source directories to be able to skip the copy.