Support for "autonumber" keyword (pro-only)

[Jostein Kjønigsen]

As seen in the documentation, the autonumber keyword is used in a single-line statement:

autonumber 2

It supports only one parameter, which may be a number or another keyword/value off.

This is probably relatively easy to add support for, and can be done independently of the other more complex pro-only syntax features.

CC: @dlasko