zeroPad string provider

[DavidUnderdown]

For 1.2 - see digital-preservation/csv-validator#122

[DavidUnderdown]

A corollary to this is that neither positiveInteger nor range currently reject values that are (unexpectedly) zero-padded (this is due to the fact that the EBNF allows 0 at any position). That behaviour can be emulated by adding regex("^[^0]") (a slightly more complex regex would be required if 0 itself were an allowed value)