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.
* :save takes a path and saves the current message part to that location
* :pipe is the same as pipe on the account page, but uses the current
message part rather than the whole email (ie :pipe gzip -d)
* Refactored account:pipe and extracted common pipe code to
commands.util.QuickTerm
* Added helper command aerc.PushError
