Support colon-delimited include paths in command-line front-end

[ender672]

Similar to the Ruby -I command, rdebug should support multiple load paths
separated by colons.

[cldwalker]

Thanks for the pull but I need a test that tests this behavior as explained in Contributing of the readme. Warning, the testing suite is passing but pretty ugly. It's on my todo to clean up.

[ender672]

Where do tests for command line switches go?

[cldwalker]

I don't see any tests for the executable and bin/rdebug needs to be moved to a class before this and most executable functionality can become testable. Blocked until refactored.

[eric-hu]

While working on #57, I came up with a scheme to test rdebug as is. I'm testing it as a separate thread to figure out how rdebug works without changing it, and ensure nothing breaks when I finally get to the refactoring step.

@ender672 your feature should be testable in the same vein. It should be straightforward to test this pull request by working off what I've written on my fork.