custom output format (3/4): added the --outformatstr option

custom output format (3/4): added the --outformatstr option
* Allows the definition of a custom output format using placeholders.
* Registers the two functions: custom_export_database() and custom_print_item()
 respectively for e_filters and u_filters.

--outformatstr only applies if --outformat=custom has been specified and
if we are querying the database (presence of the --mutt-query option)

If the first character of --outformatstr is a '!', then each entry
is checked before output: it will be full skipped if any of its fields
among those requested is NULL.

Example:
$ abook --mutt-query "Max" --outformat custom --outformatstr "!{nick}: {mobile}"
max: +336863331XX

While querying is no more mutt-centric the --mutt-query option name
has *not* been duplicated nor renamed, given its wide use.

(http://sourceforge.net/mailarchive/message.php?msg_id=27849095)