renderHook should allow returning various types

[wch]

renderHook should be applicable to the following kinds of inputs:

• tag objects
• tagLists (if we take the changes from Implement post render hooks; add hooks to tagList() #267)

And they should accept the following return values:

• tag objects
• tagLists
• NULL (to remove a tag)
• maybe atomic vectors (like strings)
• maybe regular lists

And we should have unit tests for these transformations.

[schloerke]

I believe the return values are already handled. (Yay S3.)

tagLists() are currently missing render hooks.

[cpsievert]

tagLists() are currently missing render hooks.

Just FYI, #267 will likely add them