Completely re-work colored output and tty handling.

[BurntSushi]

This commit completely guts all of the color handling code and replaces
most of it with two new crates: wincolor and termcolor. wincolor
provides a simple API to coloring using the Windows console and
termcolor provides a platform independent coloring API tuned for
multithreaded command line programs. This required a lot more
flexibility than what the term crate provided, so it was dropped.
We instead switch to writing ANSI escape sequences directly and ignore
the TERMINFO database.

In addition to fixing several bugs, this commit also permits end users
to customize colors to a certain extent. For example, this command will
set the match color to magenta and the line number background to yellow:

rg --colors 'match:fg:magenta' --colors 'line:bg:yellow' foo

For tty handling, we've adopted a hack from git to do tty detection in
MSYS/mintty terminals. As a result, ripgrep should get both color
detection and piping correct on Windows regardless of which terminal you
use.

Finally, switch to line buffering. Performance doesn't seem to be
impacted and it's an otherwise more user friendly option.

Fixes #37, Fixes #51, Fixes #94, Fixes #117, Fixes #182, Fixes #231

[retep998]

cc @Stebalien you may be very interested in the techniques used here to detect what sort of TTY is available on Windows.

[BurntSushi]

#94 is where the goods are, including some code. Arguably, that might belong in the atty crate. cc @softprops

[BurntSushi]

I filed an issue against atty: softprops/atty#10

How awesome would it be if the entire Rust ecosystem could automatically do correct tty detection on Windows? Oh boy!