This adds new functionality to :save in the message view for specifying
directories in the path. A new flag, -p, is also added to :save for
automatically creating any directories in the path that do not exist.
If the path ends in a / (e.g. "Downloads/mail/") or if the path is an
existing directory, the part's file name is the filename from the mail
header for the part. Otherwise, it uses the last element in the path as
the filename (e.g. 'blah.jpg' is the filename if the path is
'Downloads/mail/blah.jpg')
* :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
