Add 'carthage purge' to clean out the cloned dependencies

[alanjrogers]

It could be useful to clear the ~/carthage/dependencies directory so that everything is re-cloned when you next run carthage checkout.

[alanjrogers]

This'll be pretty simple, so maybe worth putting in the initial release?

[jspahrsummers]

When might you want to clean that directory?

In any case, I'm fairly 👎 on including anything in the initial release that isn't strictly necessary for basic functionality, or anything that has a workaround (rm in this case). Not necessarily because implementation will take a lot of work, but it'll be great to have a super simple initial release that does exactly what it says on the tin.

[jspahrsummers]

@alanjrogers:

When might you want to clean that directory?

[robrix]

cf #104 which will purge when migrating the directory.

[jspahrsummers]

Tentatively closing as unnecessary, though I'm open to any additional input.