add preyPart and preyStage arguments to speciesSummary()

[ahhurlbert]

When TRUE, diet summary would be broken down separately by these categories.

For example:

Lepidoptera larvae vs Lepidoptera adult, or

Prunus vegetation vs Prunus fruit

Default will be FALSE for both, in which case values will be aggregated to, e.g. Lepidoptera or Prunus.