Add a new common CLI flag to allow custom rules folders

[gillg]

Description

This change allows to use a new CLI argument wich wake up an existing pre-coded feature to allow a user to specify multiple custom rules folders.

Type of change

Select the relevant option(s):

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (optional)
• New and existing unit tests pass locally with my changes

[liyun-li]

This change is not functional dude. There is no code for processing rule_dirs.

[gillg]

This change is not functional dude. There is no code for processing rule_dirs.

Sorry but I don't understand.
I'm running scountsuite sucessfully with that code...

rule_dirs is used in __main__ line 166 in _run and 315 in new Ruleset(...) where it was already supported but just dormant.