[XML Markers] Implement generated init_xml_attribute()

[AsherGlick]

The new generated classes written in #65 need a way to parse XML values into the member variables. This will be done by implementing the now virtual function:

bool init_xml_attribute(rapidxml::xml_attribute<>* attribute, vector<XMLError*> *errors);

This function will have static code inside of it such that it is able to translate from any of the stringy attribute names, found in the xml_fields value of any of the xml docs's frontmatter, to specific logic for setting the associated class variable.

One option may be to use gperf to generate a perfect hash lookup table at compile time.

[klingbolt]

I will take this assignment