Allow modifying `_native_to_marshmallow`

This is a follow-up to #16, which was closed after introducing NewType. As explained in the following, the NewType-solution is not possible in my case, so I’d like to ask you to reconsider adding a way to make _native_to_marshmallow mutable. From the implementation it looks pretty simple to replace _native_to_marshmallow by a dynamic look-up to Marshmallow’s Schema.TYPE_MAPPING. Then I could modify the latter and marshmallow_dataclass would pick up the changes.

I would like to “register” a fixed Marshmallow field that should be used whenever a type appears in a dataclass that is converted via marshmallow_dataclass:

@dataclass
class Schema:
    color: Color

should be equivalent to

@dataclass
class Schema:
    color: Color = field(metadata={  ... some lengthy definitions here ... })

Unfortunately, I cannot replace “Color” by a NewType, because it is defined in a lower-level library. I would like to avoid having two very similar types, one from the library, and another one for use in schemas.

[MartinAltmayer]

Yes, I will try to create a PR for this.

[MartinAltmayer]

Implemented in 7794dd985168ec5d83c47c222e09d00516012628.