Add an option to fail parsing when encountering unknown Element IDs

[FreezyLemon]

As discussed in #117, there should be an option to fail parsing when encountering an Element with an unknown ID.

The behavior right now is to log a warning message, skip the Element and just move on.

One idea was a bool parameter in the public API (imagine a strict: bool param, or something similar) which would result in an error if true, and fails the parsing.

Alternatively, the logging could be changed to output an error instead of a warning, e.g. with the tracing crate.