…ler option.

* Moves `OverrideKeyword` to the more appropriate contextual keyword
  section (types.ts)
* Introduces the `CompilerOptions.noImplicitOverride` boolean option
  (types.ts).
* Moves the location for performing override assertions to
  `checkKindsOfPropertyMemberOverrides` as suggested (checker.ts).
* Adds several other diagnostic messages (diagnosticMessages.json).
* Improves the breadth of the `overrideKeyword.ts` test case.
* Adds a new `noImplicitOverride.ts` test case.

* Rename check{Implicit -> Augmented}PropertyMemberOverrides.
* Disable augment checks in ambients contexts.
* Allow override on static methods.
* Add override keyword to spec.md.
* Use chained diagnostics when possible.
* Remove hardcoded diagnotic-related strings in checker.ts.
* Split tests into separate files overrideKeyword{es5 + es6}.ts.

Test output looking reasonable, no errors seen.