luma709 greyscale

**Is your feature request related to a problem? Please describe.**
For a project I'm working on, I need to get the greyscale algorithm to use the LUMA709 colour intensities.

**Describe the solution you'd like**

Essentially this should do it:
```
parseInt(
0.299 * this.bitmap.data[idx] +
0.587 * this.bitmap.data[idx + 1] +
0.114 * this.bitmap.data[idx + 2],
10
)
```
**Describe alternatives you've considered**

I started to make the change, but realised that adding a new arg for the algorithm type will be a breaking change (`function greyscale(algorithm, cb) {`). Firstly, I was wondering if this feature is something you'd consider adding? And if so, is there is a preference for how it's implemented (i.e. a brand new colour method `function greyscale709(cb)` or something), or if adding an extra arg is the way forward.

**Additional context**

Awesome library btw! <3