Prepend an underscore to a StructName if it starts with a digit.

[avalonbits]

This situation can occur if the column name starts with an underscore prefix and the following character is a digit:

create table thing (
 _1099Form text
);

currently the generator would balk because the generated struct would be :

type Thing struct {
  1099Form string
}

and 1099Form is not a valid name (it starts with a digit).

with this change we would instead get instead:

type Thing struct {
  _1099Form string
}

[kyleconroy]

Thanks for the code! If the tests pass, I'll merge (and add a test too)