Add option to disable color output

Provides the `--color` and `--no-color` options to specifically enable/disable colored output in Strainer. Default is to output in color.

Wanted to take advantage of the `can_display_colors?` method that `Thor::Shell::Color` provides but that didn't universally remove color output. Subclassing and overriding `set_color` was the least intrusive.

This should be a fix for #34