Make possible fo value colon-separated

[g-yonchev]

Hi,
According to the spec https://tools.ietf.org/html/rfc7489#section-6.3 (and also few www examples https://mxtoolbox.com/dmarc/details/dmarc-tags/dmarc-failure-reporting-options)

... The value of this tag is a colon-separated list of characters...

When I do

let tags = dmarc('v=DMARC1; fo=0:1:s;');

getting the message Invalid value for 'fo': '0:1:s', must be one of: 0, 1, d, s.

Expect to be returned in the tags insead.

[g-yonchev]

I guess if you do

let values = value.split(/:/).map(x => x.trim());
for (let val of values) {
  if (!/^([01ds])$/i.test(val)) {
    throw new Error(`Invalid value for '${term}': '${value}', must be one of: 0, 1, d, s`);
  }
}

insead of https://github.com/softvu/dmarc-parse/blob/master/index.js#L20-L22
will fix it.