Add archive command

Adds an archive command that moves the current message into the folder
specified in the account config entry.

Supports three layouts at this point:

- flat: puts all messages next to each other
- year: creates a folder per year
- month: same as above, plus folders per month

This also adds a "-p" argument to "cp" and "mv" that works like
"--parents" on mkdir(1). We use this to auto-create the directories
for the archive layout.

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