add clapme feature to create a ClapMe impl (for command-line arguments)

[droundy]

This is a little silly, since ClapMe is so young, but I think it's relatively harmless. The idea is to make it so that one can automatically create command-line interfaces that then get translated into dimensioned quantity.

[droundy]

Sorry, I just noticed that this seems to break dimensioned when the clapme feature is not used. :(

[paholg]

Cool! The bug where this broke without the clapme flag makes me think I should probably also do tests without any features flags (ideally, it would be tested with every combination of features, but that's too many builds).