add flag to test simple modules, as described in #8

the summer is over and more time for development is available. with this pull request, i submit you a small feature that i've been using at work since months. it's a flag that allows to test simple *require* modules.

notes:

 - on my machine, `doctest <module.js> -m` will work but `doctest -m <module.js>` not. i think it's a limitation of optimist
 - i had to change the doctest main interface and thus also the way it's called in the tests, i don't remember what interface do you like more, anyway it can be changed easily
 - i added a module for manual testing. i remember you starred a project for automatic command line testing some time ago, but for the moment i won't tackle the effort of adding it here

this small feature has been very useful to me. i'm working in a big *require.js* application, i have a list of doctestable, purely-functional modules which is cycled by a bash script and fed into `doctest <module> -m`. it is clean and helps!