index: Fix variadic argument formatting

Fixes additional argument formatting.  Currently, `logger.foo('bar %s',
'baz')` will print "foo : bar %s baz".  This patch will now yield "foo
: bar baz".

Also, add tests / code-coverage reporting to prove this bug is fixed :)
Merge pull request #3 from stephenmathieson/fix/variadic-arguments

index: Fix variadic argument formatting