%F now shows the auth name or recepient name/address if the
message is from you.

+ Changes NewComposer to return error.
+ Add lib to handle templates using "text/template".
+ Add -T option to following commands
    - compose.
    - reply
    - forward
+ Quoted replies using templates.
+ Forwards as body using templates
+ Default templates are installed similar to filters.
+ Templates Config in aerc.conf.
    - Required templates are parsed while loading config.
+ Add aerc-templates.7 manual for using template data.

`go-sasl` upstream added support [1] for the obsolete [2] AUTH LOGIN
method which enables aerc to send email via servers which remain common
in the wild.

Fixes ~sircmpwn/aerc2#263

[1] https://github.com/emersion/go-sasl/commit/61afe53d
[2] https://datatracker.ietf.org/doc/draft-murchison-sasl-login/

As suggested in #268, this adds a behaviour setting for
deleting message from message viewer.

This changes the search flags for maildir and imap backends.

They now no longer use -t for searching all text. This seems to make
more sense as being the targeted recipient. I have similarly added Cc
for -c. The text search now resides under -a for all text.

This adds documentation for the config option and the command.

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.