AllowElements regex support

[theflyingcodr]

Overview

This closes #91

It adds 3 new public methods for allowing elements to be allowed using a regex pattern, they are:

policy.AllowElementsMatching(regexp)
policy.AllowAttrs("span").OnElementsMatching(regexp)
policy.AllowStyles("color","mystyle").OnElementsMatching(regexp)

This means all elements that match the regular expression provided, will be affected by the policy being built. This allows a common prefix for example to be used for custom elements and have them all be included in the sanitized output, along with any custom attrs or styles as we can do now on explicitly allowed elements using AllowElements(...)

Rules

Some rules have been implemented, would be good to get feedback:

1. If an element has been explicitly added via AllowElements those rules override any regex rules, the regex rules are simply ignored
2. Don't add multiple regex patterns that overlap, as order of execution is not guaranteed and another patterns rules could end up being matched instead of your intended. For example, rules added like AllowElementsMatching(regexp.MustCompile('^my-element-')) and AllowElementsMatching(regexp.MustCompile('^my-element-other')) would not be recommended as both will get matched and we won't know reliably which rule to use.

[shaydoc]

This will be so useful

[shaydoc]

@buro any updates on merging this PR?

[buro9]

apologies for the delay... personal life is hard, I'm mostly offline atm

[theflyingcodr]

Thanks @buro9 👍

[shaydoc]

Thanks for merging @buro sorry to hear things are hard, hope your situation improves soon.