RFC: Add method for contains(::String, ::String).

Currently, and somewhat confusingly::

    contains("Hello, world.\n", "world.") == false

So I added a method for ``contains(::String, ::String)``
that emulates ``contains(::String, ::Char)``
such that the above function call now return true.

No tests were added to be consistent with the fact that
no tests exist for ``contains(::String, ::Char)``.

Also, are pull requests the best for submitting these small changes?