Add a Config class for the different parameter setting sets

[jbueltemeier]

As mentioned in #296 it is currently difficult to deal with various parameter settings such as impl_params: bool, instance_norm: bool as well as parameters provided by the author.

In the future we should change these parameters in favour of config: GatysConfig, like

class GatysConfig(enum.Enum):
    PAPER = enum.auto()
    REFERENCE_IMPLEMENTATION = enum.auto()
    AUTHOR = enum.auto()