Ability to selectively run tests.

I've made a smallish tweak to doctest to give the user the ability to be able to only run the doctests that they want. 

It's only by modules and line numbers, so it is not perfect, but it was the simplest change to some kind of selectivity in (that I could think of, anyway).

    doctest --dt-select=Foo       src/*.hs # All tests in the Foo module
    doctest --dt-select=Foo:22    src/*.hs # Doctest on line 22 of module Foo
    doctest --dt-select=Foo:22-25 src/*.hs # Doctest between lines 22 and 25 inclusive.

I'm not attached to the implementation (esp. how I avoided pulling in some parsing deps for the arg parsing) so if you like the idea and want me to change it before merging it, just let me know.