Add node v9 color depth support

[Qix-]

Closes #98.

No need to use .hasColors() as it's a small wrapper around .getColorDepth(), which is supported in an older version of Node anyway - so it's a win win.

[sindresorhus]

I'm pretty lukewarm about this. The downside of using stream.getColorDepth() is that we have no control over it. The Node.js core team is known for making a lot of bad decisions. Additionally, the behavior of stream.getColorDepth() can change between Node.js versions, which can lead to weird inconsistencies and bugs for users.

[Qix-]

"Depth" in color terminology always means the number of bits. Usually it means number of bits per channel, but Node.js has made their own decisions about that...

I don't foresee that changing much. The benefit here is that we're no longer relying on environment variables, and custom TTY implementations can now indicate their support for colors.

Literally right after writing that last comment I snooped around some of the PRs adding support for this. I totally agree, what a nightmare - little to no thought or discussion went into adding this feature into core node.js.

Really sad state of affairs they've got going on there.

Closing. The color depth method is a timebomb.