It makes absolutely no sense to copy over the default template from aerc.conf
when the syntax is invalid and our ini parser fails.

The only valid case to do that is if the file is actually missing.

Instead of panic when compiling a regular expression, return it up

Signed-off-by: wagner riffel <w@104d.net>

Also update to the tcell v2 PaletteColor api, which should keep the chosen
theme of the user intact.

Note, that if $TRUECOLOR is defined and a truecolor given, aerc will now stop
clipping the value to one of the theme colors.
Generally this is desired behaviour though.

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.

This leads to a nasty 'mail: no address' message for each email if left
empty so the user really should enter it.

Allow styles to be layered over a base style. The list of styles to
apply is layered over the base style in order, such that if the layer
does not differ from the base it is not used. The order that these
styles are applied in is, from first to last:

msglist_default
msglist_unread
msglist_read    (exclusive with unread, so technically the same level)
msglist_flagged
msglist_deleted
msglist_marked

So, msglist_marked style dominates.

This fixes an issue where the msglist_deleted style was not being applied.