consider param missing only if param-fn returns exactly nil

template-fn already did this, but apply-template would reject
false.

(Current design assumes that param-fn will always return nil or
string?, but design may not stay this way.)
document the template-fn macro
more docstrings
template-fn* now supports a two argument overload, like template-fn
introduce one-argument version fo template-fn macro
shorten source-type keywords to just :rsrc, :file, :string, :url