Removes an assumption that specific data/keys are required by the bound
command.

Instead `Template.Option()` is used to tell the Template to error out if
and when it actually encounters a key it needs.

Unfortuneatly it seems the Template options are a little lacking, and
I don't believe can be persuaded to accept zero values or even nil
pointers as 'missing' values.

The struct in question, `PRCommandTemplateInput`, appears to have no use
elsewhere however, so was easily replaced with a map, which I hope
serves your needs for the near future at least.