Make the parsing option more robust by also parsing RGB and RGBA strings.

This option is disabled by default, and is evaluated by a single conditional within the main export. When enabled, it will also accept RGB and RGBA strings, and behave in a similar manner to parsing hex strings.

When the input is RGB, and 'a' is undefined, return the RGB string.
When the input is RGB, and 'a' is defined, return an RGBA string.
When the input is RGBA. and 'a' is undefined, use the alpha that's part of the string, and return an RGBA string.
When the input is RGBA and 'a' is defined, overwrite the alpha value, and return an RGBA string.

Behavior
The original behavior is intact, and this feature must be explicitly enabled. The interface is identical, and the results for hex strings are also identical. This introduces no breaking changes.

Why?
The intended behavior is to get an RGB(A) string from an input value. This feature still does exactly that, but accepts a wider range of input.

Use case:
Th React library material-ui can be a little unpredictable when it comes to themes. When overriding a color, sometimes it's in the form '#FFF', rgb(), or rgba(). This feature makes the function more robust, while still producing the intended behavior - an RGB(A) string that reflects, an alpha channel, or lack of.