config: allow support for groups of entries

[dominicbarnes]

From @dominicbarnes on November 17, 2016 5:8

I'm running into an issue with a project that has 2 distinct builds I'd want to run, but not ones I'd like to run in parallel. (the output of 1 feeds into the other) The best way I thought to address this was by adding an optional layer to the entries config that allows for named groups. For example:

{
  "entries": {
    "js": [ "pages/*.js" ],
    "css": [ "pages/*.css" ]
  }
}

My thought would be that if you specify no "overrides" at all, it merges all of the groups into a single list. (ie: the current behavior) But if you specify something like "css", the list of entries would only contain those CSS files. If you use a single array, without an object, the behavior would remain the same.

Copied from original issue: makojs/config#19