Exclude plugins for global pluginified files

This adds an option to concatSource to allow the exclusion of plugins
from the bundle.source. This is necessary because o plugins that are not
meant to be included in the build, a `System.set` call is added which
will not work for global pluginified outputs. Fixes #42
Merge pull request #62 from matthewp/exclude-plugins

Exclude plugins for global pluginified files