feat: deprecate custom decorators

[nknapp]

This PR marks the "registerDecorator" is now deprecated. The
decorator API exposes a lot of internal structures that
should not be part of the official API. An alternative
API can be discussed at #1574

• "Handlebars.registerDecorator" now logs a warning
• "Handlebars.registerDecoratorWithoutDeprecationWarning"
  can be used in order to omit the warning, when
  developers have read the notice.
  (The inline-decorator is using this function now)

I would value comments about whether this is the correct way to introduce a deprecation.
My thoughts:

• Pro: People only read a changelog when they have build failures. A deprecation notice in the build
  log might be more visible than an entry in the release-notes
• Contra: The warning will probably appear in the browser-console of end-users. This may not be
  what developers want. In some cases, developer my have a hard time removing the warnings,
  because they don't have direct access to the code that uses Handlebars. (like this comment suggests)
• Contra: The message may not be as visible as I intend, if it is just in the build log and in the browser-console.

[nknapp]

@wycats, @ErisDS opinions? I will fix the tests before merging, but I would like to know if what I am doing makes sense.