(GH-13) Treat function calls in the same namespace as vulnerabilities.
(GH-13) Class functions / methods tests.
(GH-13) Add get_function_name().

Allows retrieving the full function name, based on a PPI::Token::Word object.
(GH-13) Review function/method calls.
(GH-13) Update documentation.
Update module limitations.
(GH-13) Generalized "safe variables" into "safe elements".
(GH-13) Documented how to indicate safe functions.
(GH-13) Require '&' to identify functions.

Ideally, I'll implement a full-fledged tokenizer to parse SQL safe comments.
But in the meantime, there's no way to distinguish between a space inside a
hash key and a space between a safe variable and a function, unless the
function comes with an ampersand (which are less likely to exist in hash keys).
(GH-13) Add test mixing variables and functions.
Clarify documentation.
(GH-13) Implement configurable safe methods/functions.
(GH-13) Test for custom safe functions.
(GH-13) Tests for custom quoting methods.
(GH-13) Reduce code complexity.
(GH-13) Allow slightly more complex subroutines.