FEATURE: Add configuration maxSharedCacheTime and enableAgeHeaders

[mficzel]

    # enable adding of public age headers with the actual age of the cached record
    # consider disabling this if the public max-ages or s-maxage are lower than the internally allowed ages
    publicAge: true

    # the maximum value for the `s-maxage` directive for the Cache-Control header
    # set to 0 if you do not want to add the directive
    maxSharedCacheTime: 0