Adds a message indicating the user's ability to :save or :pipe a message
with an unsupported mimetype and also adds a selector widget (similar to
the tutorial).
The selector widget was previously defined in the account wizard module,
so this commit breaks it out into its own module to allow for re-use.
Further, modify the BeginExLine() function to take an argument that
pre-populates the command line, allowing functions to initiate an ex
command without executing it.
Closes#95.
Add a "new-message-bell" option to the UI section of aerc.conf. A new
hook into the message store allows the msglist widget to detect new
messages being added to the displayed list. When new messages are
delivered, and the new-message-bell option is enabled (as it is by
default), the terminal will beep.
