Allow custom attributes and added option to only allow specific elements

[dalferth]

I had the case that I have special dataset attributes in my text to be edited, which must not be lost during parsing. Since the HTML Sanitizer does not allow global options, I moved the default options to Trix.config.
In the course of this, I also added that only special HTML elements and URL protocols can be allowed.

Now something like that is possible:

Trix.config.htmlSanitizerAllowedAttributes.push("data-internal-link");
Trix.config.htmlSanitizerAllowedProtocols.push("http:", "https:");