Add feature to ignore `__Private` stuff

[YuryBandarchuk16]

@fredemmott How do you think this feature should look like?

I have a few options:

• Add flag about whether to ignore files withig __Private folder or if there is such substring in their absolute path.
• Add option with value which will look like this --ignore-directories=dir1,dir2,dir3 which will allow us to more concisely specify what do we want to generate docs for.
• The same as --ignore-directories but will also check a substring in the absolute path.

[fredemmott]

• filter on namespace, not on directory
• also exclude private methods if this option is set