Add support for NO_COLOR environment variable

[AbstractBeliefs]

When the NO_COLOR environment variable is present (with any value), disable
colour output. This can be overrident explicitly with the --color switch.

See also: http://no-color.org/

[AbstractBeliefs]

There's unfortunately no test for this: the test environment has colour suppression due to the tty detection, and because of this all colour tests have the --color switch, overriding the environment variable.

A test could be implemented by removing the --color switch and disabling tty detection with another new switch, but I'm not sure this is a helpful path to go down.

[ggreer]

I don't mean to annoy you by closing this. It's just that we seem to disagree on the usefulness of this feature. People can read my reasons here: https://news.ycombinator.com/item?id=17244313

Sorry.

[kikonen]

I surely would have liked that also this tool would have simply conformed to NO_COLOR, instead of needing to extra tricks for this case :()

Thus

alias ag="ag --nocolor"

it is