Add possibility to name shortcode different from classname

[wernerkrauss]

I e.g. have a Youtube shortcode that should be handled by a class YoutubeShortcode.

I propose to add an (optional) method getShortcodeName() to get it. I can submit a PR if you want me to.

[wernerkrauss]

Or use config system?

[wernerkrauss]

Came around this issue defining a new

class Foo extends Object implements Shortcodable

But some configuration for this would still be need for better encapsulation

[sheadawson]

Happy to merge a PR using the config system. Please update the readme too :)