Add Zero Width Character Support

[RealityRipple]

Created a new 'zerowidth' option, which is an array of characters that should be rendered with no width or side bearings.

The relevant SVG file(s) still need to exist, but a 0 wide viewport object should suffice, so long as it has some simple vertical path inside to parse.

Example use, with the Keycap character, u20e3:

20e3.svg:
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 0 36"><path fill="#FFFFFF00" d="M0 0 V 36 z"/></svg>

Gruntfile.js:

options: {
    ...
    autoHint: false,
    zerowidth: ['20e3'],
    ...
}

You can even make a character have a negative width to cause an overlap on the previous character, allowing for modifiers.