minidump-2-core: add an -o flag for controlling core output
minidump-2-core: add an -o flag for controlling core output

Always writing to stdout makes it hard to debug, and hard to use in
some script environments.  Add an explicit -o flag to make it easier.

BUG=chromium:598947

Change-Id: I79667d033c8bdc8412d3a44fe3557d65f704968f
Reviewed-on: https://chromium-review.googlesource.com/403988
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

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