Add singular_name to custom taxonomies

[bobbingwide]

Custom taxonomies can be defined using oik-types.
The logic does not currently support user entry of a singular label.
When determining the singular version of the label it can produce unwanted results.
e.g. The custom taxonomy "Block Status" ends up with a singular label of "Block Statu".

Requirement

• Add support for entry of singular name
• Display both plural and singular labels in the taxonomy list
• Generate default if not entered by user

Proposed solution

• Change admin\oik-taxonomies.php to accept the singular_name field
• Update oiktax_register_taxonomy() to pass the labels array to bw_register_custom_tags() or bw_register_custom_category().
• May need to move bw_return_singular_name() to admin/oik-types.php

[bobbingwide]

delivered in 2.0.0-alpha-20181019, but tests not complete. Closing anyway.