More flexible `when` syntax

[g105b]

Following on from #15, now it is possible to create a trigger with just the keys, it would be nice to allow variable arguments to the when function.

Examples:

$input->when("code"); // triggers when "code" is present
$input->when("code", "email"); // triggers when "code" and "email" are present
$input->when(["code" => "1234", "email"]); // triggers when "code" is "1234" and "email" is present