import (once) does not work as expected when specifying multiple src fil...

...es to a single dest.  Each dest should be run through the less parser as one unit, concatenating the src files before hand. addresses #203

This change modifies the task by sending the pre-concatenated files through the parse in one chunk, rather than separate files with a new parser instance.

Multiple dest files will be processed separately.

A single source-map is created as well for the multiple src files, so this may be a possible fix for #89 as well.
Passing all of the files pre-concatenated to the parser will also resolve issue #200, since the banner will be injected in this single operation.