default template functions

[technosophos]

Per #25, implement the Sprig functions for Bartholomew. Rather than file an issue for each function, I figured I'd break these down into small batches. (Note: I am not entirely sure all of the functions can be done with the same syntax. I am no pro in writing the Handlebars functions.)

• default DEFAULT_VALUE TEST If TEST is not truthy, return the DEFAULT_VALUE. https://masterminds.github.io/sprig/defaults.html
• empty TEST if TEST is "empty", return boolean true, otherwise return false
• all_empty A B C returns boolean true if all of the varargs are empty (OPTIONAL. Not sure if we need)
• all A B C returns boolean true if all of the varargs are non-empty (OPTIONAL. Not sure if we need)
• any A B C returns boolean true if any of the varargs are non-empty (OPTIONAL. Not sure if we need)
• coalesce LIST return the first non-empty item in the list. Extra good: if we can do Sprig's coalesce A B C.... with varargs, returning the first non-empty arg
• ternary TRUE_VAL FALSE_VAL TEST If TEST is true, return TRUE_VAL, otherwise return FALSE_VAL