Fix command line default prior bug

Previously, the default value None for l2_mult was somehow getting passed from argparse all the way into the objective function.  I changed the argparse default to fix this.  I also changed the L1/L2 prior defaults everywhere to 0.0 rather than a mix of 1.0 and 0.0.

Note: for max-length-5 Pseudo-Korean, having 0.0 priors results in somewhat different weights on each run.  I think this is one of those cases where that's supposed to happen (because the relevant constraints other than *Struct are never violated?), but it's worth noting anyway.