Add option to detect symlinks in cookbookdirs as cookbooks about between-meals

[jaymzh]

• In Changes::Cookbook.initialize we should for each file store if it's a symlink
• Add a config option for want_symlinks_to_count_as_cookbooks
• Then in Changes::Cookbook.meaningful_cookbook_file? if the option is set and it's a symlink and it's in a cookbook dir, return true.

[mikedodge04]

#74

[davide125]

@jaymzh @mikedodge04 is there anything left to do here?

[jaymzh]

Closed by #74