New field: "Expires"

[joker314]

This is a new field which will be introduced by securitytxt/security-txt#183

This field indicates the date/time after which the data contained in the "security.txt"
file is considered stale and should not be used.
The value of this field follows the format defined in section 3.3 of {{!RFC5322}}.

This field MUST NOT appear more than once.

I'm thinking we should be able to accept a Date object, or an RFC5322 compliant timestamp. If something else is given, we can try to feed it to the Date constructor (so if an integer is given, it will be treated as milliseconds since 1970).

[lirantal]

Sounds good to me.

[bkimminich]

Added the directive via #70 but it accepts anything currently,