Support grunt sync --reposList=

[zepumph]

From #405 and while working on https://github.com/phetsims/phet-studio/issues/11. I wanted to get set up with website repos, but didn't have an easy way to support this, since grunt sync is hard coded to active-repos.

[zepumph]

@samreid can you please give this a spot check? It is a little confusing that if you use grunt sync --repoList=active-website-repos from friction/, then the list of repos that are updated will be friction, perennial, and all active website repos, but I don't think it is worth the complexity of making the options or logic mutually exclusive. The implementation centers around a goal of being able to easily and thoroughly update repos provided. I didn't want to risk excluding something from that notion. Furthermore, it is really graceful, for example if you have working copy changes in perennial, it will just tell you.