Formatter: new get_formatter(type)

Return Formatter of proper flavor, else NULL (for plain)
mon: use new new_formatter() for mon dump; allows all formats
Formatter: remove mistaken assert in dump_stream()

dump_stream was complaining if there was a pending string (basically,
an item still open), but this is standard usage for the dump_*() operations;
you start one, and then it's closed when the next one comes along.
Remove the assert and let print_spaces() close the existing element
and indent-if-pretty, as usual.