Add remember/recall functions to kara-templater

[nielsm]

Add a set of "remember"/"recall" functions to kara-templater. This is a very commonly needed function, that usually needs some code line hacking to get in a useful way.

Possible implementation in kara-templater.lua:

tenv.remembered_values = {}
tenv.remember = function(name, value)
    tenv.remembered_values[name] = value
    return value
end
tenv.recall = function(name)
    return remembered_values[name]
end

[nielsm]

(In [55bdd38]) Add a remember/recall suite of functions to kara-templater. Closes #1277.

[nielsm]

(In [23510dc]) Merge kara-templater remember/recall functions from 2.1.9. Updates #1277.