This is done by routing the generation of HTML into a temporary directory,
and checking its diff agains the existing public/ when running tests.
Fixes#237
VERBOSE will be set by TRAVIS (but can also be set by user to diagnose slow
builds). Using an env var so that it automatically propagates to all the
sub-scripts and tools without having to pass it through tools/build explicitly.
gofmt no longer supports `tabs` and `tabwidth`, but we still require
exactly-4-space tabs to preserve the narrow layout on gobyexample.com,
so re-implement this functionality with sed.
