Use parsers.parseColor() in border-color parsing, like background-color

[bawjensen]

My attempt at fixing #184. Please feel free to use this code however ends up most useful.

[asamuzaK]

I think we don't need v.toLowerCase() === 'transparent'.
It's already resolved by css-color.

To make sure, add test like below somewhere around test/parsers.js#L64

  it('returns color for transparent keyword', () => {
    let input = 'transparent';
    let output = parsers.valueType(input);

    assert.strictEqual(output, parsers.TYPES.COLOR);
  });

And before test/parsers.js#L248 i.e. it.todo('Add more tests')

  it('should output transparent keyword', () => {
    let input = 'transparent';
    let output = parsers.parseColor(input);

    assert.strictEqual(output, 'transparent');
  });

[bawjensen]

I had effectively copy/pasted this from the background-color logic. Should I update that too, to keep in sync? (I've proactively updated it, but as a separate commit that should be easy to revert if needed)

thanks for the review!

[asamuzaK]

LGTM