Enable spell checking

Steps to get started:

1. `git clone project`
2. `yarn install`
3. `yarn link` 
4. `yarn link devtools-linters` in debugger.html
5. uncomment retext-spell [here](https://github.com/jasonLaster/devtools-linters/blob/master/natural-language/preset.js#L16)
6. add words we want to the [dictionary][d2] 

[d2]:https://github.com/jasonLaster/devtools-linters/blob/master/natural-language/dictionary.txt#L7