[Danger-prose] Improvements for danger + proselint

[jessesquires]

Follow-up from #333

• Re-enable prose.check_spelling, which seemed to not work before

• We'd also want some words to have a canonical spelling (for example, does the compiler suggest "fixits" or "fix-its"? Do we invoke the compiler on the "command line", "commandline", or "command-line"?).

• We need the bot to be able to recognize or just completely ignore Liquid {% tag syntax %}.

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Danger: http://danger.systems/ruby/

Proselint: https://github.com/dbgrandi/danger-prose