Build: Add ESM files + babel updates.

[ryan-roemer]

• Add ESM es/ directory and package.json:module field. Fixes Infra: Add ESM output. #491
• Enables webpack tree-shaking. Fixes many spectacle dependencies bloats my bundle #490
• Update to babel 7
• Refactor all es7 method arrow funcs (foo = () => {}) to real methods (foo() {}). (Just ones that are class methods, not generic arrow funcs).
• Update one-page files with modern includes and catching up to the full Spectacle API.
• Add example to lint
• Add builder to run all our different builds concurrently for speedup.
• Update travis config

/cc @kenwheeler @carlospaelinck

[carloskelly13]

Should we also pass context? I know theming uses it. (Same with other components)

Or alternatively

constructor() {
  super(...arguments);

[ryan-roemer]

Released in https://github.com/FormidableLabs/spectacle/releases/tag/v4.2.0