Fix double formatting

Fixes two bugs that led to formatting the same source twice in many
cases. The first one – the leftover "if exactly one source" special case
from before the rewrite of command line options – kicked in in the
common case 'ceylon run ceylon.formatter source/', while the second one
– the missing "reset currentSources to null after redirecting" – led to
redirections being formatted twice, once with redirection and once
in-place – very bad!

This also fixes part of #34 ;-) (now takes 6 seconds instead of 12)