Re-implement Css Customization

[nickwinder]

Fixes : https://github.com/PSPDFKit/react-native/issues/219

Details

As explained in https://github.com/PSPDFKit/react-native/issues/219#issuecomment-488771585 due to limitations with file permissions in UWP it was necessary to rework the CSS customization. Therefore now the viable solution is to provide a native option.

In this PR I have reverted the previous CSS work and added the option to start the PSPDFKit react native package with a Uri parameter which will be passed as the CSS resource. Within this CSS resource, it's possible to set the main colors of the toolbar. As before.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, and samples/Catalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).
• After merge ensure all links in README works.