16 million (a.k.a Truecolor) support

[eljobe]

First I'd like to say that I am a big fan of your rgbterm work. I have recently been working on developing a way to determine what level of color support a terminal emulator has. I have been hacking together https://github.com/eljobe/terminal (although, maybe it should have a less collision-prone package name.)

I'm wondering if you would be interested in a pull request which uses that functionality to determine what level of color is supported by the terminal, and then automatically, inside the rgbterm methods either:

• Use the full 16 million colors of Truecolor (i.e. not convert to the 256 color codes) if Truecolor is supported.
• Use the 256 color map (i.e. rgbcolor's current behavior) if 256 colors are supported.
• Convert even further down to 16 colors (ansi or basic color) if only 16 colors are supported (e.g. Terminal.app that is the default terminal on most Macs.)
• Totally ignore the escape codes, if no color support is found in the terminal (obviating the need for programmatically enabling/disabling color)

Let me know what you think. Otherwise, I'll probably just end up ripping off the API you've provided to your users and wrapping calls to rgbcolor to implement 256 color support.

[aybabtme]

@eljobe sounds good to me, I'd be happy to merge that PR!