feat: remove empty non-boolean attributes

[dylanarmstrong]

Summary

This causes sanitize-html to remove empty attributes that are known to be non-boolean. It provides an exhaustive list of all known attributes taken from: https://html.spec.whatwg.org/multipage/indices.html#attributes-3

• User controlled via options.nonBooleanAttributes
• Enabled by default, and can be disabled by passing in nonBooleanAttributes: []
• Can handle all empty attributes via nonBooleanAttributes: ['*']

Closes #123

What are the specific steps to test this change?

Test with empty attributes such as:

<input checked type="checkbox" />

Will not remove anything.

<input checked form type="checkbox" />

Will transform into the first example.

What kind of change does this PR introduce?

• [ ] Bug fix
• [x] New feature
• [ ] Refactor
• [ ] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [x] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [x] The changelog is updated
• [x] Related documentation has been updated
• [x] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

[dylanarmstrong]

Thanks for feedback, updated documentation.