Support for NO_COLOR and CLICOLOR / CLICOLOR_FORCE env variables.

[dclause]

This PR add respect for env variables regarding ansi support:

• NO_COLOR (see the sepcs)
• CLICOLOR and CLICOLOR_FORCE (see the sepcs)

It solves the issue #22

The PR also adds a readme update as well as tests regarding the env variable support and precedence rules. The support order and rule of precedence for the env variables are given in the code comments.