Add support for styling header line with telephone-line

[MikeTheGreat]

Turns out the header line can be set up using the same syntax as the mode line.

Since you've done all the hard work to style the mode line it wasn't too hard to reuse all your hard work to format the header line.

Thank you for doing (and sharing) all your hard work!

[dbordak]

Oh, cool, I sorta thought it would work with the header line. I'm unsure of the implementation, though. Following this, you'll disable any header line the user already has defined; if someone wanted to use telephone-line with their existing header-line (not using the header-line support in telephone-line), it'd be troublesome. I think instead it should be a separate minor-mode.

[MikeTheGreat]

Ok, I added a commit (6f52f52) which separates the header-line feature out into it's own, separate mode.

Thanks for this feedback, by the way - I wasn't 100% happy with how it worked the other way but wasn't sure how to enable header line support. The separate minor mode is a good way to go (and way easier than I expected it to be)

BTW, if this ends up making into the 'master' branch I'd be happy to update the documentation to mention this somewhere (into the README.org, yes?)