dartdoc: add a command line flag to exclude build time from the output

Documentation will often be in a SCM of some type. I ran into a case where I wanted to make some tweaks and check out the diff of the docs.

With the current config, *all* docs are updated on each build, which makes navigating changes difficult--especially if only a handful of comments are changed.

A command-line flag to exclude the timestamp would be ideal.
