Add support for addProtocols method in Config

[ishults]

Currently there is no way to add protocols to whitelists as allowed in JSoup here: http:https://jsoup.org/apidocs/org/jsoup/safety/Whitelist.html#addProtocols%28java.lang.String,%20java.lang.String,%20java.lang.String...%29

It would be help if we could add protocols via something like:

allow('a') {
    protocols attribute: 'href', value: 'http'
    protocols attribute: 'href', value: 'https'
}

or

allow('a') {
    protocols attribute: 'href', value: ['http', 'https']
}