Allow weekday prefix for most date formats

This is implemented now using the "skip" parser field, indicating
to skip the first N characters. This also avoids a recursive parse
in one more case (more efficient). This simplifies the state machine
a little bit, while the rest of the code needs to properly account
for the value of the skip field.

Also allow whitespace prefix without penalty.

Modify the test suite to psuedo-randomly add a weekday prefix
to the formats that allow it (all except the purely numeric ones).

Author: klondikedragon