Add behavior to allow users to use CM addons that come with LT

Opted to not use triggers for load-js and load-css since that will require
adding them as configurable app behaviors which no one is asking for.
Also extended load-css to optionally take multiple paths like load-js does
and updated load-js and load-css docs to reflect multiple optional paths.
Merge pull request #1779 from cldwalker/cm-addon-behavior

Add behavior to allow users to use CM addons that come with LT