Add support for output file suffixes (versions, for example)

In our project, we are required to deliver our project files (.css, .js, etc) with a version number, i.e. project-1.33.7.css. At the moment, the lesscss-maven-plugin does not seem to support that, and can only customize the folder to output the files to.

This (small) commit adds support to add a common (optional) suffix to output files, for example:

    <configuration>
        <outputFileSuffix>-${project.version}</outputFileSuffix>
   </configuration>

This will output all input files as <original-filename>-1.33.7-SNAPSHOT.css, for example.

I've added a unit test and an integration test for this parameter, although the unit test itself isn't very pretty (lot of copy / paste). But, it was that or refactor the entire suite, don't think that'd help.

It would be nice if this could be integrated into an official release, atm we're getting by with a local repository installation.