Make empty arrays and dictionaries render compactly even when indent > 0

[lihaoyi]

Fixes #499

We take a similar strategy as we do for adding commas: rather than calling renderIndent immediately after opening a new array/dict and at visitEnd, we instead set an indentBuffered flag and defer the renderIndent until later. Then each flushBuffer call can render the indent where necessary, and visitEnd can perform some additional checks that the collection is not empty before rendering the final indent

Added some basic tests to ExampleTests.scala, since some of this behavior is probably worth showing to users in the documentation