Make template model be any interface type

[arnodel]

This means models can be bespoke structs.

[codecov-commenter]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files          10       10           
  Lines         410      410           
=======================================
  Hits          400      400           
  Misses          5        5           
  Partials        5        5           

Impacted Files	Coverage Δ
templates.go	100.00% <ø> (ø)

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[arnodel]

@keighl any chance this could be merged into master? I have been using this fork fork for almost 5 months.