When changing templates, it's can be pretty time-intensive to
repair all test fixtures.
This change instruments CLI tests with two flags, `-update` and
`-pretty-diff` that control how test fixtures are diffed. When the
`-update` flag is set, the tests fixtures are overwritten as tests
execute. The `-pretty-diff` flag causes the full text of the fixture
to be printed on mismatch.
Oliver Gould