Feature request: Field format support

[thehappycoder]

Some PDF fields need to adhere to a specific format. For example, if you manually try to enter to a field, Adobe Reader may yell The value entered does not match the format of the field = "99 99 99". This format can be anything, so it's impossible to hardcode in the code.

At the moment, pdf-fill-form just enters whatever string you provide to it to the pdf, and it's not formatted. However, when you enter it manually via Adobe Reader, it gets automatically formatted -- spaces are added automatically.