add prettier code formatting and linting for import/export values

[mwcz]

This adds prettier code formatting. It will be applied when eslint --fix is run. It also adds eslint-plugin-import which lints imports and exports. For example if you try to import { fooBlah } from "./lib.js" and lib.js doens't have a fooBlah export, eslint will error.

It also adds husky and lint-staged to allow linting to happen when a file is staged in git. There was already a precommit npm script but precommit requires husky and (optionally) lint-staged to be installed in order to work. npm itself doesn't have a precommit script.

[blalasaadri]

I would recommend adding the examples directory to the ESLint scan as well - turns out, there are quite a few issues there. So, that would mean that in the package.json we would change the scripts definition to:

"scripts": {
        "..."
        "eslint": "./node_modules/.bin/eslint src examples",
        "eslint-fix": "./node_modules/.bin/eslint --fix src examples",
        "..."
}

[mwcz]

@blalasaadri I also wanted to add the examples dir, but it required cleanup I wasn't ready to do yet (time constraints). Part of the problem is the lib dir, which has files we definitely don't want to lint. I'll update this PR with a .eslintignore file to exclude lib.

[mwcz]

Thanks @blalasaadri !

[blalasaadri]

It seems the Travis build is now failing because it's merging this branch onto master and the main_text.js file there doesn't yet follow the new guidelines. I've looked into it and the only problem I can't seem to be able to fix is that the variable text is unused; maybe @JeffreyBrettColeman (who added that variable) can help here? My change with everything working except for that is at https://github.com/blalasaadri/hero-engine/commit/f43159358d824a35f566f13b715fbc9b649c68ec.

[mwcz]

@blalasaadri @JeffreyBrettColeman I applied @blalasaadri's fix and also removed the line length linting rule (prettier handles that for everything but comments, which don't seem worth enforcing).