added the ability to use custom colors by extending the default color…

[vectorfrog]

Twix currently only recognizes the default colors, so if I use a custom color that I've defined in the tailwind.config.js file, it will not be merged. This code change allows a developer to extend the colors that Twix uses by adding a list in their config.exs file: `config :twix, colors: ["primary"]

Example of current behavior:

tw "text-primary-500 text-green-200" "text-primary-500 text-green-200"

Example of new behavior: tw "text-primary-500 text-green-200" "text-green-200"