Add option to colorize unread messages.

With listnormal_unread and listfocus_unread two new options have
been added that make it possible to customize colors for unread
articles.
Fix linking of utils::to_string

A new template instance with type unsigned int was needed
Allow json-c lib named either "json" or "json-c"

With version 0.11, the json-c library (https://github.com/json-c/json-c/wiki)
renamed their pkgconfig file from "json" to "json-c".

This way, either name is accepted and the website hinting on failure is
preserved.

See also: https://bugs.gentoo.org/show_bug.cgi?id=467354
Merge pull request #77 from eroen/allow-json-c-lib-named-either

Allow json-c lib named either "json" or "json-c"
Merge pull request #75 from ISF/fix_to_string

Fix linking of utils::to_string
Merge pull request #74 from pks-t/unread-color

Add option to colorize unread messages.
wrong bookmark argument order when bookmarking from macro (fixes #79).
Merge remote-tracking branch 'upstream/master'