Add AddAptRepo class with spec

[andis]

This pr adds a contrib class AddAptRepo.

[sometimesfood]

Thanks for the pull request, this looks great.

However, there's a couple of small things I would like to see changed before I merge this:

• bundle exec rubocop shows a number of warnings. Could you please fix them?
• Please remove the core_file and core_kernel member variables and use fakefs and a simple Kernel.stub in the tests instead.
• The attr_reader for wright should be private.

The documentation is not parsed correctly by yard and has a few other issues:

• The @return tag simply renders as 'self' which is probably not what you wanted. In this case I would simply specify [void] as a return value since the actual return value should not be part of the public API here.
• The class itself does not have a return value. Please move the @params, @return etc. to #call or #initialize and keep only the short explanation and the usage examples in the class comment.
• The examples should have two separate @example tags so they are rendered independently. Also, could you add a #! line so that the context from which self can be passed as the 'wright' argument is clear?
• The type annotations for the parameters are missing.
• The example needs to be indented by two spaces. The options in the option hash should be documented using the @option tag.

Just run bundle exec yard and have a look at the resulting docs and you'll see where the generated documentation is falling short of your expectations.

[sometimesfood]

Also, can you please make sure that all of your code is run at least once in the tests?

The coverage report (COVERAGE=1 bundle exec rake test) shows that there are some lines that are not covered by the specs.

[andis]

Work in progress (docs):

• @return tag
• move @params, @return etc. to #call or #initialize
• @example tags
• type annotations for the parameters
• indentation
• option hash

[andis]

• fix rubocop warnings
• remove core_file
• remove core_kernel
• increase test coverage