Allow ModelOptions to have extra parameters

[rmb938]

Closes #397

ModelOptions can now have extra parameters. This only applies to params that start with '_'

Usage:

>>> class Foo(Model):
...     class Options:
...         _test = 123
... 
>>> Foo.Options._test
123
>>> Foo._options._test
123

[rmb938]

@bintoro Can I get some feedback on this and possibly get it merged & released?

[lkraider]

You already can set your own options class (just inherit from ModelOptions SchemaOptions):

class FooOptions(SchemaOptions):
    def __init__(self, test=None, **kwargs):
        super(FooOptions, self).__init__(**kwargs)
        self.test = test

class Foo(Model):
    __optionsclass__ = FooOptions
    class Options:
        test = 123

[lkraider]

Thanks for the PR, let me know if somehow your use case is not catered by the custom class.

[lkraider]

Thanks, this has been merged now.