Add template functions as html render option

[brutella]

This change adds the field Funcs to the HTMLOptions struct to specify additional functions for every html rendering.

I use it to specify a translation function T to localize text.
Since every html request needs a different translation function T (because users have different language settings), I have to specify it for every html page rendering.

I've also changed the value type of Options.Funcs from []template.FuncMap to template.FuncMap.
I couldn't find any reason why it has to be a slice – or do I miss something obvious?

[brutella]

I still feel like the Funcs option should be of type template.FuncMap instead of a []template.FuncMap.

Are there any plans to release a new major version, where this breaking change could be introduced? Should I create a separate issue to track that?

(I'm still thankful for this library though. ❤️)

[unrolled]

This looks good, thanks for the contribution! No plans on a new major version yet, but I've got a document tracking breaking change requests now 😉