Port newer color readability functions from TinyColor

[DavidHarvey]

The "port" of TinyColor is either old, or modified, such that readability methods return unexpected results. This simply ports the latest readability methods (and necessary complementary methods, like getLuminance).

As an example, take the background color #0066cc and the foreground color #fff. This combination passes all WCAG contrast checks, except for AAA on normal sized text (See: https://webaim.org/resources/contrastchecker/?fcolor=FFFFFF&bcolor=0066CC)

Before:

-> tinycolor.isReadable('#0066cc', '#fff')
<- false

After:

-> tinycolor.isReadable('#0066cc', '#fff')
<- true

-> tinycolor.isReadable('#0066cc', '#fff', {level: 'AAA', size: 'small'})
<- false