Improve the output from the code completion metrics tool

This CL does three things.

First, it makes the output options more fine grained to allow us to get
the information we're after without getting information that we don't
care about at the moment.

Second, it prints the unconditional output in a more compact tabular
form so that it's easier to read.

Third, it adds support for outputing a high-level comparison of multiple
experiments to make it easier to gauage the impact of a change.

Change-Id: Iee8b1eb126de43cb08c27a1b60036090fffc0421
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174941
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>