Add support for using ranges for RGB

[Erovia]

This patch adds support for controlling continuous ranges of RGB LEDs.
Helper functions for split boards are also available.
New functions are also present in the documentation.

[drashna]

The *_master and *_slave` functions should probably be wrapped by ifdef's, so that they don't do anything on non-split keyboards.

Also.... I don't think they will work on i2c split boards, because of how the code is set, but I could be wrong.

[Erovia]

The *_master and *_slave` functions should probably be wrapped by ifdef's, so that they don't do anything on non-split keyboards.

Also.... I don't think they will work on i2c split boards, because of how the code is set, but I could be wrong.

I thought about it, but wasn't sure what condition to use. "SPLIT_KEYBOARD" is not yet present in every split's rules.mk, is it?
Anyway, they only control the first/second half of the LEDs, so I don't think they cause much trouble.
(or maybe I'm just naive :D)

I could only test the code on a Let's Split rev2, which afaik uses i2c and it works fine.

[Erovia]

@drashna, does this PR needs anyting else?

[mechmerlin]

Thanks!