A1 Image Use Case

In the docs it says

```
A1 and RGB30 do not yet support getImageData or putImageData.
Have a use case and/or opinion on working with these formats?
Open an issue and let us know! (See #935.)
```

I'm happy to provide my use case to argue in favour of adding more support for 1bit. I work with a waveshare eink display and it requires a 1bit bmp to paint the screen image.

## Steps to Reproduce

```js
var Canvas = require('canvas');
var canvas = Canvas.createCanvas(200, 200);
var ctx = canvas.getContext('2d', {pixelFormat: 'A1');

var imageData = context.getImageData(0, 0, width, height); // Throws Fatal Error Null Pointer (not implemented)

// or I might want to use canvas.toBuffer('image/bmp')
```