Internal: Create registerPlural method to provide an easy way of working
with plurality

- There are four groupings of plural / singular methods in the new API:
  table(s), row(s), column(s) and cell(s). We need to provide similar
  methods for each, often leading to code duplication. To help reduce
  this code duplication I've created a new `registerPlural` method which
  allows both the singular and plural to be derived from a single
  function automatically. The plural form is given and wrapped up by a
  container function which extracts a singular return.

- Note that not all singular methods can use this approach, for example
  row().data() provides extra functionality over rows().data() in that
  it can be used as a setter as well as getter.
Internal: Small enhancement to registerPlural to correctly deal with
same instance returns.