Add contractions for spell checking

[bbondy]

```// NB: This is to work around tinyspeck/slack-winssb#267, where contractions
// are incorrectly marked as spelling errors. This lets people get away with
// incorrectly spelled contracted words, but it's the best we can do for now.
const contractions = [
  "ain't", "aren't", "can't", "could've", "couldn't", "couldn't've", "didn't", "doesn't", "don't", "hadn't",
  "hadn't've", "hasn't", "haven't", "he'd", "he'd've", "he'll", "he's", "how'd", "how'll", "how's", "I'd",
  "I'd've", "I'll", "I'm", "I've", "isn't", "it'd", "it'd've", "it'll", "it's", "let's", "ma'am", "mightn't",
  "mightn't've", "might've", "mustn't", "must've", "needn't", "not've", "o'clock", "shan't", "she'd", "she'd've",
  "she'll", "she's", "should've", "shouldn't", "shouldn't've", "that'll", "that's", "there'd", "there'd've",
  "there're", "there's", "they'd", "they'd've", "they'll", "they're", "they've", "wasn't", "we'd", "we'd've",
  "we'll", "we're", "we've", "weren't", "what'll", "what're", "what's", "what've", "when's", "where'd",
  "where's", "where've", "who'd", "who'll", "who're", "who's", "who've", "why'll", "why're", "why's", "won't",
  "would've", "wouldn't", "wouldn't've", "y'all", "y'all'd've", "you'd", "you'd've", "you'll", "you're", "you've"
];

const contractionMap = _.reduce(contractions, (acc, word) => {
  acc[word.replace(/'.*/, '')] = true;
  return acc;
}, {});