Exposed the placeholder text for title and content...

...as a public property which can be modified after the editor is setup. This change is needed for https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/373

/cc @diegoreymendez 
I'd guard this inside `if (_titlePlaceholderText != titlePlaceholderText) { ... code goes here ... }`.

Not a deal breaker, but it's a good addition.