support @internal tag

[paulgibbs]

I'd like to update the wp-cli commands to not create posts for functions/classes/etc marked as @internal unless an override flag is passed to wp-cli.

When generating documentation from PHPDoc comments it is RECOMMENDED to hide the associated element unless the user has explicitly indicated that internal elements should be included.

An additional use of @internal is to add internal comments or additional description text inline to the Long Description. This may be done, for example, to withhold certain business-critical or confusing information when generating documentation from the source code of this piece of software.

https://github.com/phpDocumentor/phpDocumentor2/blob/develop/docs/PSR.md#78-internal

[paulgibbs]

Going to go ahead and add this later this week unless I get an objection soon :)

[rmccue]

:+1:

[Rarst]

This seems sound in spec, however WP's use of @internal is (as usual) inconsistent.

On this, #37 and anything alike I am thinking towards standardizing behavior on default everything and opt-out options.

The primary goal of parser is to extract data as abundant as possible from source processing, narrowing that data down is post-processing stage.