Added automotive provider for nl_NL

[MarcelRobeer]

What does this change?

Added an automotive provider for Dutch (nl_NL), which generates license plates for cars, motorbikes and other road vehicles according to https://en.wikipedia.org/wiki/Vehicle_registration_plates_of_the_Netherlands (format 6 through 10, covering 1999 to present).

What was wrong

faker dit not include license plate formats for the nl_NL locale.

How this fixes it

• Added an nl_NL folder in faker/providers/automotive/
• Added an __init__.py file in the nl_NL folder, with the same format as faker/providers/date_time/es_ES/__init__.py
• Added license plates for motorbikes (license_plate_motorbike()), cars and other vehicles (license_plate_motorbike()), and override license_plate() to generate 10% motorbike plates and 90% other road vehicle plates (according to official CBS statistics approx. 10% of road vehicles are motorbikes)
• Added test cases for license_plate(), license_plate_motorbike() and license_plate_car()

[fcurella]

Thank you!