Support context.letterSpacing

## Issue or Feature
I would like to be able to adjust the kerning/letter-spacing of fonts when drawing text on node-canvas. In a browser this is possible by setting canvas.style.letterSpacing to something like "-3px". But canvas.style I believe does not exist on node-canvas.

(BTW we love node-canvas, it is an amazing piece of software.)

## Steps to Reproduce

Interactive example of it working with browser-based Canvas:

https://jsfiddle.net/hg4pbsne/1/

Example code:
```
canvas.style.letterSpacing = '-5px';
ctx = canvas.getContext('2d');
ctx.textAlign = 'center';
ctx.textBaseline = 'middle';
ctx.font = '4em sans-serif';
ctx.fillText('World', can.width/2, can.height*3/4);
```

This is not possible to reproduce with node-canvas as there is no canvas.style property exposed.

## Your Environment

All versions of node-canvas.

## My Suggestion

We add a non-standard ctx._letterSpacing parameter to node-canvas that exposes this functionality?

The issue names no source file, test, or entry point. Start by comparing the linked browser example with node-canvas text drawing, then clarify the API and rendering semantics for letter spacing; done means the requested spacing works when drawing text.

Written by the indexing model from the issue text.