Add support for integrity and crossorigin attributes #98

The external_scripts and external_stylesheets arguments for Dash for R currently lack support for attributes permitted in Dash for Python:

• integrity
• crossorigin

These should be added for parity with the Python implementation of Dash. The HTML5 specification provides for a rich set of possible attributes for <link> and <script> tags:

<link> attributes:

• as
• crossorigin
• disabled
• href
• hreflang
• importance
• integrity
• media
• referrerpolicy
• rel
• sizes
• title
• type
• methods
• prefetch
• target

<script> attributes:

• async
• crossorigin
• defer
• integrity
• nomodule
• nonce
• referrerpolicy
• src
• type