Support for color values with percentage representation ?

## Issue or Feature

The current implementation looks like ignoring color value with percentage representations `rgba(100%,100%,100%,1.0)`. It is valid css color value according to [this](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value) document. Is there any plan for supporting this or am I overlooking something ?

A library I am using outputs with this format and I am stuck here :S

## Your Environment
* Version of node-canvas (output of `npm list canvas` or `yarn list canvas`): canvas@2.4.1
* Environment (e.g. node 4.2.0 on Mac OS X 10.8): Mac OS X 10.4.3