Improve `solo init` logic with Berkshelf and Librarian-Chef

Currently `knife solo init` defaults to `--berkshelf`. Then if berkshelf gem is not installed but Berksfile exists, a warning is issued while cooking.

Rules to select the default action (first one that matches):
   1. If Berksfile or Cheffile is found, default to the corresponding option (prefer Berkshelf). We don't override existing files, so this might only write .chef/knife.rb (relates to #239).
   2. If berkshelf gem is found, default to `--berkshelf`. I.e. generate Berksfile etc.
   3. If librarian-chef is found, default to `--librarian`. I.e. generate Cheffile etc.
   4. Default to `--no-berkshelf --no-librarian`, but maybe display some information to promote the use of one or the other?

We should also improve the documentation in respect with these options and knife.rb (especially `knife[: berkshelf_path]`) configuration. Maybe put some examples to the wiki?