Add ability to append to rules/macros

[mstemm]

Add the ability to append to rules/macros, like we already do with
lists. For rules/macros, if the object has an append: true key, the
condition value is appended to the condition of an existing rule/macro
with the same name.

Like lists, it's an error to specify append: true without there being an
existing rule/macro.

Also add tests that test the same kind of things we did for lists:

• That append: true really does append
• That append: false overwrites the rule/macro
• That it's an error to append with a prior rule/macro existing.