Allow using multiple formatters

Formatters (and output files can now be chained):

    behave -f plain -o plain.txt -f json -o result.json -f pretty

Empty stream assumes that sys.stdout will be used. Note, that -f option without a -o parameter should be specified last (not sure whether it is fixable - this seems natural to me)

Fixes #47

Sorry for spamming with PR, had to rebase onto master