This adds the ability for per-account signatures in the accounts.conf
config file. The signature is added to emails in the editor at the
bottom of the email. This includes when forwarding, replying to, and
composing emails.

There are two config options: signature-file and signature-cmd. The
former allows a signature to be read from a file and the latter allows
an arbitrary command to be executed to return the signature.

The config options have been documented in aerc-config

This adds the event type as well as the command implementation, but no backend
supports it yet.

Add an onUpdateDirs handler. This is used to invalidate the dirlist and
redraw with the correct number of recent/unread/total messages is shown.

A config option and formatting options are provided.

This patch adds search behaviour to allow searching in the body of the
messages, the entire text (body + header), and searching just the from
header.

Allows to set `ui.spinner=` to a string which is then split by
`ui.spinner-delimiter=` (Default: comma) instead of having a hard coded
animation.

This implementation doesn't use INIs capabilities to split strings as
it trims whitespaces breaking the default animation.

Signed-off-by: Paul Spooren <mail@aparcar.org>

Usage:
    :prompt <prompt> <command...>

Displays the prompt on the status bar, waits for user input, then
appends that input as the last argument to the command and executes it.
The input is passed as one argument to the command, unless it is empty,
in which case no extra argument is added.