Add header support to text format and matching feature

> It's hard to find an explanation what this patch actually does. I don't
> want to have to go through threads dating back 4 months to determine
> what was discussed and what was actually implemented. Since you're
> already using git format-patch, just add something to the commit message.
>
>
> It appears that these are really two separate features, so perhaps they
> should be two patches.

Thanks for the feedback, I've split cleanly the two patches, simplified the
tests and tried to explain the changes in the commit message.

> Also, the new header matching mode could probably use more than one line
> of documentation.

I've improved the documentation, let's me know if it's better.

It seems like cfbot is not happy with the way I'm sending my patches. The wiki
has some good advices on how to write a patch but I couldn't find anything on
how to send it. I've used

git send-email -v3 --compose --to=... --in-reply-to=... HEAD^^

here but I'm not sure if it's correct. I will see if it works and will try to fix
it if it's not but since it runs once a day it may take some time.