Document use of generic ignore option

[paulo-ferraz-oliveira]

Related to issues #507 and #488.
Related to PR inaka/elvis_core#137.

Can be used to update the Rules wiki. Once the related PR is accepted.

Note: remove references to ignore_functions from the Wiki + move no_tabs, line_length and no_trailing_whitespace to elvis_text_style.

Tentative documentation follows.

The "ignore" option

Module-level rules implement a generic ignore mechanism that allows skipping analysis in elements of your choice.
It suffices to add the ignore list to your rules, as per the example below.

-elvis([{elvis_style, invalid_dynamic_call, #{ ignore => [elvis_core]}}]).
-elvis([{elvis_style, no_debug_call, #{ ignore => [elvis_result, elvis_utils]}}]).

You can add the exceptions using the following syntax:

• whole module: ignore => [mod]
• same name functions: ignore => [{mod, fun}] (available for elvis_style-based rules only)
• module, function and arity: ignore => [{mod, fun, arity}] (available for elvis_style-based rules only)

[paulo-ferraz-oliveira]

Updating Rules wiki with this content, since it's been merged to master already.