commands/msgview/save and commands/msgview/pipe now use case
insensitive comparisons to determine if the part is encoded as base64
or quoted-printable.
* :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