Allows the outgoing command to contain arguments and flags

Add a command for removing attachments from a composed message. Syntax
is :detach [path], with path being an optional argument specifying the
path of one existing attachment. If no path is specified, the first
attachment is removed.

Allow users to add attachments to emails in the Compose view. Syntax is
:attach <path>, where path is a valid file. Attachments will show up in
the pre-send review screen.

Tab completion currently only works on commands. Contextual completion
will be added in the future.