Add runtime configuration warning

Add an implicit parameter to the mount function with sensible default to configure what run time warning. Typical example we should be able so catch:

• <input onClick={f}/> instead of onclick
• <input clazz="c"/> instead of class
• <inpu/> instead of input
• <div>foo</div> instead of foo.toString

[olafurpg]

It would not have to be an implicit parameter, a regular parameter with default value is also fine.

[olafurpg]

Aah, I misunderstood the role of the config.

I’m not sure either whether “hot fixing” typos is a good idea. I can see it being occasionally convenient but it in the bigger picture it might allow you to be sloppy, which is bad IMO.

I guess it’s best to leave this up to users. This design

trait Config {
  def transformElement(a: Any): String
  def transformAttributeKey(k: String): String
  def transformAttributeValue(a: Any): String
  ...
}

is incredibly flexible and it could be used in a ton of unexpected way to create developer/instrumentation tools. The power could be abused to hot fix typos, but I still think we should go with that.