Add --mask-directive option

[estum]

Description

This PR adds the --mask-directive option similar to --hide-tag but for directives.

The following example will mask @!parse directive with YARD::Tags::NilDirective class to suppress any callbacks:

yardoc --mask-directive parse

Also, the option allows to suppress warnings on unknown directives in different contexts.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[estum]

Honestly, the second use-case was a main target for me and I explain why:

I have a project depended on swagger-yard gem which defines several custom YARD tags & directives to declare docs for RESTful API in controllers by using YARD syntax.

The same files may contain documentation for regular ruby methods, so I have tried to make a custom definitions to be ignored by yardoc.

Whilst the --hide-tag option has already supplied to do it with custom tags, directives won't to be tamed and were keeping to annoy with unknown directive warnings. So, there is the dumbest solution I found.