flag: add BoolSlice flag parser
flag: add BoolSlice flag parser

BUG=None

Change-Id: Ib154c9912ecf012f77d722d5c1e4d306eca89666
Reviewed-on: https://chromium-review.googlesource.com/c/1498685
Auto-Submit: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go