* custom output format: fixes and enhancements

* custom output format: fixes and enhancements
 - support for the {groups} placeholder
 - support for escaped characters (\n, \t, ...)
 - removed the leading empty line from the output
 - fix segfaults when an empty or a special format string
 	was provided
 - referenced --outformatstr in --help
 - code cleanup: use of built-in ITEM_FIELDS and standard_fields[]
 	to fetch standard fields numbering

* vcard output: removed the leading empty line
	(which is mutt-specific)

* colors: added the has_colors() ncurses check
	before actually initializing colors.

* Changelog, RELEASE_NOTES, AUTHORS and THANKS files updated