Prints out how long it takes to generate each section.

It's done using a DurationReporter helper class that prints starts
counting when constructed and prints the duration when destructed.

Typical usage:

function do_something() {
    DurationReporter duration_reporter(title);
    // Do something.
}

Change-Id: I87134d9a1b003300384376c242a3c034a46244c4