Add prettier

[fathyb]

Rebased on #39, please merge it before this one.
Same as segmentio/analytics.js-core#89

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This PR adds prettier-eslint, which is just prettier with its output piped to eslint --fix. js files are formatted using eslint rules, while md and json files only using prettier.

husky is added to setup a Git precommit hook, and lint-staged to only check staged files. This will make sure every commit is properly formatted and passes the lint test. This can be bypassed by using the Git commit --no-verify option.

A format script is added to run prettier on all files.

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https://segment.atlassian.net/browse/LIB-413
cc @f2prateek