PerchAPI_Base: no-index support

Hello,

Not sure whether the indexing mechanism is going to change in v4, but if it remains the same, it may be a good idea to have the no-index attribute supported by default. This would give developers greater control on what gets indexed when building sites using add-ons

e.g. Perch Shop seems to index all product fields including custom fields and the no-index attribute has no effect.