cli: extract option splitting and use it for --compiler (#746)

[darrinholst]

Thought I'd take a crack at #746 while I was dealing with : splitting for the formatters. The compiler isn't as easy to unit test as the formatters was since the result of the split is used internally to the config builder plus it actually requires the module so you can't just send anything you want to it...like a windows absolute path. So I added an OptionSplitter utility that both the --format and --compiler can use. There are no new tests for --compiler because of the reason above, but the existing feature should ensure there are no regressions.

The splitting itself almost turned out to be simple with negative look aheads and look behinds until I found out that js regex doesn't support look behinds. That's why there's still a little bit of logic to smash those back together.

[charlierudolph]

Looks great! I'd like to clean up the test duplication a bit but otherwise this is good to merge.

[charlierudolph]

Thoughts on converting this to something like

// The following is within the describe block for OptionSplitter

const examples = [
  { 
    description: "doesn't split when nothing to split on",
    input: '../custom/formatter',
    output: ['../custom/formatter']  
  }
  //...
]

examples.forEach(({description, input, output}) => {
  it(description, function() {
    expect(OptionSplitter.split(input)).to.eql(output)
  })
})

[darrinholst]

I went ahead and changed it. I'm kind of indifferent either way. The individual it blocks were already pretty terse, but this way does remove what boilerplate there was and gets to the point of what you're testing.

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