Write report to stdout even with --html-report

This way I can see the output in Jenkins console even when generating HTML and I don't need to run `diff-cover` twice.

Alternatively if folks like having it be silent when `--html-report` is specified, then we could add a command-line option like `--write-to-stdout` or whatever, but that seemed perhaps overly complex, and if users don't like getting output on stdout from `--html-report` they could `> /dev/null`.

Cc: @aconrad
Kind of out-of-scope of this PR, but wouldn't it be a tad safer to use a `with` statement when opening the file so it gets flushed / closed as soon as we're done writing to it?