Add ignore action

[krassowski]

For now, it is just a manually checked set. Ideally, we would be able to add it to the dictionary on the fly, so that we get suggestions on the ignored words (e.g. JupiterLab → JupyterLab), however Typo.js does not seem to be capable of that.

Given that Typo.js is also quite slow with suggestions, it might be worth considering an alternative implementation:

• Atom's spellchecker looks promising, but it might require node environment (so would not be an option).
• hunspell-asm appears even better - especially given the option of merging multiple directories (which would allow to load basic English + medical English - would be a dream feature for me!).

I also improved some corner-cases for suggestions (nothing that would be noticed by the user, just exceptions being thrown without proper handling) - see the first commit.

[ijmbarr]

Thanks for all the contributions to the jupyterlab_spellchecker extension!

As you might have noticed, I'm not that active with the extension any more. Mostly due to a change in jobs, I no longer use jupyterlab that much. I was wondering if you would be interested in taking over the extension? It's not much work, and it would mean you wouldn't have to wait for me to approve your PRs.