Fix colors not appearing in non-tty environments

[fiznool]

Fixes #609, #616, #669

Also maybe fixes #648?

[Darkhogg]

This should be require('colors/safe').enabled = true to avoid String.prototype modifications. Actually, adding it on line 13 with just colors.enabled = true should be more than enough.

[indexzero]

We want to try to add test coverage here before merging this.

[fiznool]

@indexzero can I help here? Would really like to get this merged in.

[indexzero]

@fiznool absolutely. Can you write a test that spins up a process running winston in a non-TTY environment with colors enables and ensure that it works with colorize: true and colorize: false?

[fiznool]

@indexzero I've added test coverage for this and rebased/squashed commits.

[indexzero]

Awesome work @fiznool!

[fiznool]

Thanks for the speedy merge!

[indexzero]

I'll always make time for anyone who takes the extra time to write tests and contribute to keeping the quality of the project high. Again: well done.