Add support for custom entity definitions in ParsingOptions

[nihilox]

I’m using the roxmltree library version 0.18.0 to parse some epub, but I encountered a problem with entity references. Some of the files do not have standard <!ENTITY ...> settings, such as the common ones

.add_entity("nbsp", " ")
.add_entity("copy", "©")
.add_entity("reg", "®")

This causes the parsing to fail. I think it would be more convenient and flexible if the library could provide a way to inject additional entity definitions in the ParsingOptions, instead of modifying the file content.

Is this possible to implement? Thank you for your consideration.