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.

This allows selection of a tab using its index. It attempts to parse the
given argument as a number, if it fails then it uses it as a name.

Also supports relative indexes using prefixed + or -.

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.

Aerc will keep track of the previous 1000 commands, which the user can
cycle through using the arrow keys while in the ex-line. Pressing up
will move backwards in history while pressing down will move forward.

This patch sets up the trigger config section of aerc.conf.

Each trigger has its own function which is called from the place where
it is triggered. Currently only the new-email trigger is implemented.

The triggers make use of format strings. For instance, in the new-email
trigger this allows the user to select the trigger command and also the
information extracted from the command and placed into their command.

To actually execute the trigger commands the keypresses are simulated.

Further triggers can be implemented in the future.

Formatting of the command is moved to a new package.