Cache contents of config file

[benthemonkey]

Placing a console.log inside config.js shows that we're loading the config from scratch once per rule. We should be loading the config once per invocation of sass-lint.

I'm referring to using sass-lint from the CLI, and specifying a config file. e.g. sass-lint -c .sass-lint.yml

[DanPurdy]

I actually noticed this while working on the CLI recently and adding some of the extra options. This is definitely something I'd like to address when I overhaul the CLI and add error codes etc.