feat: strict mode for parsing

[barredterra]

Resolves #51

Introduce a strict parsing mode which defaults to True. This maintains the current behavior of raising en error when an unknown element is encountered. If set to False, the parser ignores unknown elements.

Example:

Document.parse(samplexml)  # parses only known elements, raises TypeError otherwise
Document.parse(samplexml, strict=False)  # parses what it can, returns without errors

Implementation:

Since the TypeErrors are usually raised in the from_etree method, I added the strict parameter in all paths leading there.