Set MA_BASE_SCHEMA_CLS in Document / EmbeddedDocument

[lafrech]

This changes allows to set a base marshmallow schema class in the Document / EmbeddedDocument template.

It removes the complicated logic used to pass schema parameters (check_unknown_fields, meta, params) along the as_marshmallow_schema / as_marshmallow_field calls.

The former implementation passing params allows twisted cases like passing a specific parameter to a schema deeply nested in the document when calling as_marshmallow_schema, but in a way that is absolutely not practical: it takes a hugs unreadable dict as input.

In real life, schema parameters such as unknown are most often the same for all the application, so it makes sense to factorize them in a base schema.

Corner cases can always be addresses with this implementation. It might be a little bit more cumbersome, but it should be feasible and even more readable.

This PR also fixes the cache in as_marshmallow_schema that was broken in 3.0.0beta. The fix is in the first commit (set meta unknown in cache modifiers) but is obsoleted by subsequent commits.

[coveralls]

Coverage decreased (-0.2%) to 95.557% when pulling 32bd9b8 on base_schema_cls into 8a0aaf9 on master.