Add flag --no-color

[breml]

Fixes: #17

[breml]

Added tests for --no-color flag. The commit does also include a little refactoring of the tests. To clean the tests even more I suggest to search the output (with or without colors) for the same expected strings like:

• Found Non-Approved license
• github.com/fake/package

A solution would be needed for the error level, because it is differently written. With colors, it is 4 upper case letters, without colors it is the full word in lowercase.

Additionally I suggest to sort the output of GetLicenses (https://github.com/frapposelli/wwhrd/blob/master/cli.go#L69 and https://github.com/frapposelli/wwhrd/blob/master/cli.go#L106) before returning it, which would make testing much easier, because the output would be always the same (which is currently not the case due to the random order of traversal of maps in Go).

[frapposelli]

thanks!