Add support for Electron 5 and above
Add support for Electron 5 and above

Starting with Electron 5.0.0, the function `webFrame.setSpellCheckProvider` has a different signature, in order to support asynchronous spell checkers. For more information on this change in Electron, see https://github.com/electron/electron/pull/14032.

This diff updates `SpellCheckHandler` to use the new interface if running on Electron 5.0.0 and above. However, it still checks the spelling synchronously like before.

If running on Electron versions below 5.0.0, the `SpellCheckHandler` still works.

Closes #144.

paulcbetts/electron-spellchecker,paulcbetts/electron-spellchecker