Add textlint

[avigoldman]

We should have some linting on the API reference content to help fix common errors:

textlint rules:

• alex
• max-comma (max 3)
• terminology terms: SMTP, Transmissions API etc., SparkPost, MTA, Message Systems, Momentum, Port25, TPS and other protocols, JSON, PHP and other library languages,
• write-good options: weasel, adverbs, so, illusion, cliches
• spellchecker
• stop-words (This might be too aggressive for us)