Add public constants for available changeFreq

[JohJohan]

Is your feature request related to a problem? Please describe.
It would be nice to have constants for changeFreq available options are: always hourly daily weekly monthly yearly never

Describe the solution you'd like
In Url.php add constants for changeFreq

public const CHANGE_FREQ_HOURLY = 'hourly'

We could then also have the setter validate if the set is valid?

[JohJohan]

Let me know if this is something we want, i can setup a pull request.