Add a 'community' option in the 'modernizr' config object.

When building modernizr, the only way to add community options to the build is to have them in any `*.js`, `*.css`, `*.scss` files and set the `parseFiles` options to `true`.

This PR add a `community` option in the configuration allowing you to add non-core detects options without needing the files to be parsed. When `parseFiles` is set to `true` and `community` is defined the options are merged together.