Feature: Add '--' custom arguments support.

[ryan-roemer]

Fixes FormidableLabs/builder-react-component#27

Also adds a little CI check to have builder itself run the CI test task.

This is take two of this PR as the complex scenario is: should I add the -- to the next underlying task. We now have a resolution of this which is:

1. If a builder task, then yes
2. Otherwise no

You can see this in action with the builder-react-component where I pass --help all the way down to karma instead of having it run the test like normal.

$ formidable-react-component-boilerplate $ builder run test -- --help
[builder:proc:start] Command: builder run npm:test -- --help
[builder:proc:start] Command: builder run test-frontend -- --help
[builder:proc:start] Command: karma start node_modules/builder-react-component/config/karma/karma.conf.js --help
Karma - Spectacular Test Runner for JavaScript.

START - Start the server / do a single run.

Usage:
  /Users/rroemer/scm/fmd/formidable-react-component-boilerplate/node_modules/builder-react-component-dev/node_modules/.bin/karma start [<configFile>] [<options>]

Options:
  --port                <integer> Port where the server is running.                            
... SNIPPED STUFF ...                                  

/cc @waynelkh @coopy @chaseadamsio @baer @exogen @boygirl

[ryan-roemer]

NOTE: Coveralls decreasing coverages is expected as I actually added a new test file with tests for new behavior. There's still a ways to go to get existing code tested...

[coopy]

👍