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.

When filter is active we want to use store.results instead of
store.uids, since we are dealing with a subset of the uids. Otherwise
any methods involving len will have undefined behavior.

Signed-off-by: Kevin Kuehler <keur@ocf.berkeley.edu>

Currently we /dev/null stdout, if it is a tty.
The checkConfigPerms function, as well as the error print were incorrectly
writing to stdout and therefore weren't visible to most users.

Allows the outgoing command to contain arguments and flags

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.

Adding an attachment, switching to a different tab, and switching back
to the review message caused the "filled space" in the review message to
disappear, since there was one too many rows in the layout.

A bug corrupting long In-Reply-To headers has been fixed upstream in
go-message:
https://github.com/emersion/go-message/issues/44