Add flag to `rm` for permanent deltion

[grantseltzer]

We can implement the /permanently_delete endpoint: Link

[grantseltzer]

~~Currently being blocked by #43~~

[waits]

That sounds great. Maybe rm --permanent? Note that it's only available to business accounts; I think the API will just return an error otherwise.

[grantseltzer]

@waits Ah, I actually didn't notice that. I thought it would just delete instead of moving to the trash bin for anyone. I can certainly still implement it, I'm sure dropbox would like to have it.