#347: Added filters to taxonomy based on get_terms args.

Fixes #347.
@rmccue Should we use a different filter name here, since `get_terms` isn't using the same list of filter parameters available to the posts collections? Current behavior is slightly inconsistent, but based on the below I could see adding
```php
$valid_vars = apply_filters( 'json_tax_query', $valid_vars, $taxonomy );
```
above the `foreach` and using `json_tax_query_var-` for the individual queries.

Prior art:

- The Posts class uses `json_query_vars` to filter the list of valid vars, and then `json_query_var-$var` to filter each one as it is set on the query object.
- The Users class uses `json_user_query` to filter *all* the provided filters (no whitelist), and as there is no whitelist, there is no per-filter var filter defined.