Add :fixed_numeric_not_null

I created a new option for `contains`: `:fixed_numeric_not_null`

It does the same thing as `:fixed_numeric`, but does not generate numbers that starts with a zero. I'm unsure of the naming of the option - rename it as you see fit. :)

For example, if length is set to 5, it will generate numbers between 10000 and 99999.