Modify existing taxonomies using Taxonomy class

[jjgrainger]

Currently, the Taxonomy class will not run register_taxonomy if the taxonomy already exists (line 189). Therefore, a taxonomy cannot be modified using the class.

Will need to investigate a way to

• Allow the class to modify a taxonomy
• Use existing arguments over defaults if taxonomy exists (see StackOverflow answer)
• Register changes later in order to overide existing taxonomy

[jjgrainger]

Fixed in #80