[bug] Report size not validated in URI with ruf/rua

[niftylettuce]

Related to #8, the value after the exclamation mark in a ruf/rua size value is not validated currently.

[niftylettuce]

Ref:

• https://github.com/domainaware/checkdmarc/blob/master/checkdmarc.py#L1108-L1140
• https://github.com/trailofbits/dmarc/blob/554bd015745d6e5f51cbce01608c6659aa1db96b/lib/dmarc/uri.rb