Integrate spell checking as a part of Travis CI checks

[joohoi]

@jsoref wrote a promising tool for automating this: https://github.com/jsoref/spelling and an example of how to integrate this into CI automation can be found here.

This issue is created so the information doesn't get lost now that #89 is merged. As that's where the discussion initially took place.

[jsoref]

I now have a GitHub action for this: https://github.com/check-spelling/check-spelling

You can see it on https://github.com/microsoft/terminal/commits