Support to parse color from rgb[a] string format

[luatnd]

As in CSS or react-native stylesheet, we often use rgb/rgba format.
This pull request is to support RGB string with alpha channel supported, eg:

• rgb(100,100,100)
• rgba(255, 255, 0, 0.2)

Also a feature request here:
#21

[coveralls]

Coverage decreased (-5.0%) to 93.578% when pulling 8513d57 on luatnd:master into cd18393 on onury:master.

[luatnd]

I have added some test cases but this pull did not pass the coverage check!