Added support for 'validate' command and all of Flyway's options for commands

Hi, I extended your plugin a bit and wondered if you'd like to include those changes.

I added support for 'validate' command, as well as extended the range of supported options to the entire set for the supported commands.

There's also some level of error handling - you can no longer configure options that are not used by a given command (so that any misspelled option is picked up) and options required by Flyway to work are checked by the plugin itself.

If you want to use Flyway's default values for some options you can now just omit these options in Gruntfile and they won't be present in command line (previously that would result in setting them as "undefined").

I updated the README file to reflect all the changes.