Allow colored output on MinTTY consoles on Windows

[RyanGlScott]

Currently, the Idris REPL doesn't print colored output when you run it in a MinTTY console like Cygwin or MSYS:

This makes me sad.

The reason this happens is because due to a quirk in the way MinTTY, calling isATTY in it will always return False. But we shouldn't just use isATTY to determine if we're in a console that supports colors! We can detect the presence of MinTTY pretty reliably by inspecting the file descriptor that MinTTY uses for its standard output. The minimalist mintty library accomplishes this, so I've added a dependency on it.

Now we can have colored output on MinTTY:

Yay!

[Melvar]

Is this not a bug in the other software?

[RyanGlScott]

The fact that isatty doesn't work on MinTTY is something which the maintainer has declared as wontfix.

GHC has to work around this too by employing the same trick.

[melted]

Travis was just a time-out. Restarted that job.

[melted]

@Melvar It's a quirk, if it's nothing much we can accommodate those. Like with the nixOS hashbang.

@RyanGlScott It would be nice if the check could be folded into the tty check in Util/System.hs, so it doesn't leak into Main.hs and Output.hs.

[RyanGlScott]

@melted, good idea, done.

[melted]

Thanks for the PR!

[jfdm]

I think this user facing change should be mentioned in the CHANGELOG. Please can it be updated.

[RyanGlScott]

@jfdm, oops, that totally slipped my mind. I've opened #3648 to atone. :)