Allows adding the created and modified dates to the output. Doesn't
change the default behavior.

Example:
$ my_activity.py --begin=01/01/19 -f "{created} {modified} {url} {title}"
2018-12-19 2019-01-09 https://crbug.com/916521 Tests do not resolve libbrillo-libchrome dependency properly
2018-12-05 2019-01-14 https://crbug.com/912312 authpolicy: Cache authentication data

BUG=None

Change-Id: Idfe8aa9553dfb8f9774ee4915e85b6167e6fb75b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1511252
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>