Support rel and referrer policy attributes

[PaperStrike]

Motivation and Context

The request to send by an element typically relies on two attributes, rel and referrerpolicy:

• For <a> and <area> activations: follow the hyperlink | HTML Standard.
• For <form> submissions: plan to navigate | HTML Standard.

This PR adds support for this two attributes to Pjax's default trigger.

You may have noticed the spec didn't mention referrerpolicy attribute on <form>s. Yes, it hasn't achieve a standard, yet. I still add its support in Pjax, cause:

1. WHATWG members consider the lacking of this attribute an "oversight" and tend to add the support. See Add referrerpolicy to <form> whatwg/html#4320.
2. No reasonable case conflicts with this attribute. (Even rare within very non-conforming plugins / websites.)
3. Bypassing this for <form>s requires a more complex algorithm.

How Has This Been Tested?

Sorry, I don't have a plan for adding default trigger's test cases, yet.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no bug fix and new feature but improvements)

Checklist:

• My change requires new tests.
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.