Normalizers should be cacheable with CacheableSupportsMethodInterface

[Korbeil]

Second hand on #52

With Symfony 4.1, a new interface CacheableSupportsMethodInterface was given to explicitly say that:

Marker interface for normalizers and denormalizers that use only the type and the format in their supports*() methods.
By implementing this interface, the return value of the supports*() methods will be cached by type and format

@see https://symfony.com/blog/new-in-symfony-4-1-faster-serializer

It seems that all the normalizers generated by Jane are eligible to this cache and given that on a serious project we have tenth of normalizers, we can get an important performance boost.

Fixed some stuff that wasn't working.

[Korbeil]

All ready to merge 🎉

• So I rebased since PHP CS update fixes #55 as been merged (and fix all tests).
• Added a test for this new setting
• Added some documentation

[joelwurtz]

Why doing an option for cacheable support ? It should be always enable imo, no need for an option here. And if we must support old versions of serializer, then this interface should be implemented only if it exist

[Korbeil]

After quick talk with @joelwurtz & @bastnic we decided to keep the option but default value would be null.
If value is null, we will check if cacheable exists and will add it if it exists. And you can also force to true or false if you always want it or not.