Add form difference prose for Kalos Pokémon.

To do:

- Add form prose for Mega Evolutions
- Update old Pokémon's form prose
- Nail down the style a little:
    * Do I want to capitalize the form moniker?  e.g. "Vivillon's
      Pattern depends..." or "Trading [...] does not affect the Pattern
      it will have..."
    * What about when I'm actually writing a form name?  e.g. "The Fancy
      and Poké Ball Patterns..."  (This is what I do at the moment.)
    * Change all remaining instances of "Forms only affect appearance"
      to "Forms only differ in appearance".  It sounds better and fits
      with the other opening sentences better.