Feature: Item sets

[timkurvers]

This PR implements the bare minimum for item sets. In the future we should look into rendering out benefits gained as well.

Listing item sets:

Viewing an individual item set:

An item part of a set:

Currently a bit unsure where the quality / color from an item set comes from. Thoughts @fallenoak?

[timkurvers]

Fixes #12.

[fallenoak]

Recapping an earlier discussion: since looking up records in Item.dbc currently involves a worst case of iterating over every record for each item in the set, let's stick to the approach of using the first item in the set to define its quality. We can improve upon this once we get the new DBC parser up and running.