Add .Site.Copyright support

[jamesericdavidson]

As mentioned in #111, .Site.Copyright is a Hugo variable.

Will you accept a PR which adds this string to the footer? I have one ready to go.

[526avijitgupta]

We already add copyright with a year. If someone wants to update this in the future, we'll encapsulate it into the footerHTML.

[jamesericdavidson]

We already add copyright with a year

The <footer> uses {{ now.Year }}, which is not applicable to anything made in the past or future, hence the interest in adding it:

• If I have some content written before 2022, and not subsequently updated, the copyright date is not applicable
• Unless I update all content at 00:00 January 1 2023, the copyright date will not be applicable next year

I understand that you don't want to support this, just throwing my hat into the ring. Have a good one.