Ignore unknow value

[MoOx]

I've spotted a bug on myth with rebeccapurple.
It's added to late (after rework-color-function) so doing stuff like color(rebeccapurple alpha(80%)) just don't work.
So I just move the plugin before rework-color-function but got an issue for my test should be able to disable rebeccapurple node support & it's related how this plugin don't handle unknown color.
I found that it's related to color lib that doesn't warn when it fail to parse a color so I made a PR to fix that.

Qix-/color#26

When this get merged we should be able to detect that & probably just do nothing when we don't recognize a value.
Maybe we can just warn the user ? Any idea how to make a friendly warning ?

[MoOx]

(Note the issue is: it give rgb(0,0,0) for unknow color)

[MoOx]

PR has been merged & exceptions are now thrown starting 0.7(.1). So we will need to handle that right after the merged of the #4
I'll prepare something tomorrow morning.