Add support for buster rootPath configuration options

The buster.js configuration supports a rootPath, which specifies the root directory in which source files live.  If this is specified, buster resolves all paths against this root.

buster-coverage currently doesn't support this rootPath option, so that coverage tests have to specify files using different paths, or based on where the buster command is being originally run from.

These commits by @gugu and @rowanbeentje add support for the rootPath option.