Introduced configuration file

   All feed properties that were previously set via crossbow-set(1) are now
   defined by a configuration file. The configuration is searched in
   ~/.crossbow.conf, ${XDG_CONFIG_HOME}/crossbow.conf and
   ~/.config/crossbow.conf

Unified all binaries into one

   Having a configuration file allows to have a single binary named
   crossbow(1), since all other binaries were just required to access the
   feeds configurations.

New persist file format

   In previous versions of crossbow, the feed updates were tracked in the
   same files as the configuration.  Since the feed configuration now lives
   in the configuration file, the feed persistence file is used uniquely to
   store updates.

Merged 'print' and 'pretty' logics

  Granted that 'pretty' is just a glorified 'print' that should use a special
  format.

Many minor improvements

Updated man pages