Breaking(expandable): add ngdoc, standardise option names and format

In particular changes are:
- all options moved from `gridOptions.expandable.xxx` to `gridOptions.expandableXxxx`
- `rowExpandableTemplate` changed to `expandableRowTemplate`
- defaults added for row height
- error check added for `rowExpandableTemplate`, module is disabled with an error
  message if not present
- overall `enableExpandable` option provided
Bother.  Yes, you're right.  The PR is in, but the code isn't actually there.  Hmmm.  I guess it'll throw an error until it's there, which is pretty much what it did before I added that extra code.