React Katex Versions Save

Fixed:

• Added React 16 as a supported version of the lib @jamesplease

Exporting using "module" package.json field may break in some cases (including default create-react-app setup), so we decided to remove it and use the default "main" field.

Huge thanks to @dudko, our new maintainer!

Changes:

• Improve documentation and usage examples, by @dudko
• Better error handling and katex update, by @dudko

• Added parsing error handling with a renderError prop (#9)