Add support for customisable colors.

Customising the colors using the environment variables.
Fix t/harness.t after the previous commit.

It got broken due to assumptions of the colors staying the same.
Less abbreviated env var names.

Per:

https://github.com/Perl-Toolchain-Gang/Test-Harness/pull/12
Merge pull request #12 from shlomif/master

Customisable colours