Allow filterable dropdown to accept custom inputs

[chilleyQSI]

FilterableDropdown widget can accept choices which are not strictly enforced. In order to do this, choices are input under gooey_options rather than as a kwarg to the add_argument() method.

Example

@Gooey()
def main():
    p = GooeyParser()
    p.add_argument('not_strictly_enforced',
                   widget='FilterableDropdown',
                   gooey_options={
                       'choices': ['apple', 'banana', 'carrot']})

    p.add_argument('strictly_enforced',
                   widget='FilterableDropdown',
                   choices=['apple', 'banana', 'carrot'])
    ...

Not sure if the same should be added to the simple Dropdown class or if it makes sense to leave that as strictly enforced only.