New features, bugfixes, better testing

Resolves #128 (bench with neovim) via #127
Fixes other bugs in the bench script (e.g. vim packages support, -o doesn't work)
Resolves #140 (non-percent saturation/luminance for SASS & SCSS)
Resolves #151 (color overflows)
Refuses to render invalid color specifications (e.g. color: rgb(0, 17, 136 / 100%))
Adds much more comprehensive test suites for css and html, including items that should not be colored.
Implements HSL (Hue, Saturation, Lightness) angles (e.g. color: hsl(167grad, 30%, 60%))
Implements HWB (Hue, Whiteness, Blackness) function coloring
Supports decimals and spaced values (e.g. color: rgb(0 16.99 136))
Support legacy HTML3-style colors (e.g. <body text="blue"> and <font color="#abcdef">)
Probably some more

Benchmark testing showed no appreciable increase in cost to run, but I don't like how inconsistent its stats are.

To implement hwb(), I abstracted hsl2color() to hsl2rgb() in order to retain the RGB channels. While at it, I also converted the other similar functions so there's now only one place where RGB is converted to hexadecimal representation.

ap / vim-css-color

New features, bugfixes, better testing #187