Add item template function

[boidolr]

To further customize item creation it can be useful to have an actual function.
Regardless of whether you'd want to add this functionality the first four commits might be a nice refactoring for the structure of templater.

[javve]

This is a really nice feature @boidolr! Thank you so much and sorry for taking almost two years before I merged it 😳

[DavidBruchmann]

great, first time since two years I use it, and have to do directly an update 😂

[devgianlu]

I believe this broke .values(newValues), see #698

For a workaround see https://jsfiddle.net/smvhdaq3/

[adriancb]

Further to @devgianlu's comment, I believe this regressed #438

[Antonio-Bennett]

+1 to @adriancb. This isn't gonna get fixed anytime soon :(