Let's make plugin possibilities more functional

[mihaylovich]

Let's give to plugins to export custom rules and not only build your own infrastructure to check files.
For this:

1. Extract interface ISqlRule to TSQLLint.Common
2. Add method to IPlugin with default implementation
   IDictionary<string, Type> GetRules(){
   return new Dictionary<string, Type>();
   }
3. Read and use rules from plugins