Remove dependency on escape-string-regexp

[Chalarangelo]

As chalk is used by thousands of people daily and provided that escape-string-regexp is a very simple package, I went ahead and removed the extra dependency from package.json, replacing it with an inline function. Feel free to close, if you believe this could introduce problems down the line, as it is an opinionated change.

[sindresorhus]

No thanks, but you should take a look at #300 where I propose a better solution ;)