support output plugins

Currently support for Markdown output is hard-coded in RendererMain.java. With a little work this should be generalizable so that users can provide their own renderers. Define a Render interface or base class, change renderMarkdownHeader to renderHeader, etc. DItto for MarkdownUtil - abstract the API to FormatUtil or some such, change markdownCellFormat to cellFormat, etc. Then maybe add a --renderer option, or an attribute, where the user could provide a Java class.

Maybe this is already possible using the proto output? Unfortunately I know nothing about protobuf, and the protobuf rules for Bazel are effectively undocumented (follow the links, there's nothing there there).

Start by reading RendererMain.java and MarkdownUtil to map the hard-coded Markdown APIs, including renderMarkdownHeader and markdownCellFormat. Review the existing proto output and the linked Bazel protobuf documentation before deciding how user-provided renderers should be configured. Done means output rendering is extensible beyond Markdown and the proposed renderer option or attribute is defined and tested.

Written by the indexing model from the issue text.