Fix for issue #18 (configuration options on the command line).

In the Application DSL, you can now do:

 val myconfig = config('myconfig, 5)
 MyKernel('someconfig -> myconfig)

This will cause the config option 'someconfig for MyKernel to be set
to 5 by default.  It can then be modified by supplying "-myconfig <value>"
on the command line.