add plugin tutorial section on adding a new type with corresponding formats and a transformer

[gregcaporaso]

• update nw-align chapter and others to talk about associating artifact classes, rather than semantic types, with actions
• update types of types to talk about artifact classes as a main entry point
• add section on discovering artifact classes
• add section on creating new artifact classes, with their associated semantic type, formats, and transformers (the code we'll develop is here
• review all semantic type/etc discussion, to make sure it's consistent with how we're using the terms in this section - in particular, make sure that artifact class and semantic type are differentiated.