fix: Set unpkg default to feather.min.js

style: Fix formatting

fix: Remove unused polyfills and update build config

Removes polyfills for `Object.assign` and `Set` which, as far as I can tell, are unused in the
codebase. Updates `webpack` from 3 to 4. Replaces `babel-preset-es2015` with `babel-preset-env`.
Reduces the minified bundle size (`feather.min.js`) by **9.3kB** (72.4kB to 63.1kB). Does not change
any functionality of the library.

feat: Add `archive` icon

feat: Add SVG sprite support (feathericons#319)

fix: Fix semantic-release deploys

Updated .travis.yml to match the example provided in the documentation
https://semantic-release.gitbooks.io/semantic-release/content/docs/recipes/travis.html

feat: Add tags to tags.json

feat: Add more tags

feat: Add `linkedin` icon

Closes feathericons#77

feat: Add more tags to tags.json