Prior to this commit, the composer was based on a map[string]string.
While this approach was very versatile, it lead to a constant encoding / decoding
of addresses and other headers.

This commit switches to a different model, where the composer is based on a header.
Commands which want to interact with it can simply set some defaults they would
like to have. Users can overwrite them however they like.

In order to get access to the functions generating / getting the msgid go-message
was upgraded.

The data was passed around as a string for some reason, which led to time
precision loss and wrong dates being displayed.
Simply pass the time as is to fix that.

Fixes #316

Due to headers being essentially free text, we constantly run into issues
with parts of the body being interpreted as headers.

Remove the ability to overwrite headers to avoid that, while keeping the ability
to specify headers in the template files.

Fixes #383

Hi. This adds a template function to convert a time to the local time zone. And modifies
the default quoted_reply template to use it and show the time zone when formatting the
timestamp of the quoted message.

Previously, the quoted message timestamp was UTC and it would format it without the time
zone. And I thought it might be a little confusing or weird to some normal people when I
email them and I don't want normal people to be confused or think that I'm weird.