Add a --markdown flag to GN to be able to output help in markdown.

With this change, we can now generate a markdown page containing all
of the built-in help from GN, so that it will be visible on the web.

The changes to GN to support markdown are, so far, minimal, but the
resulting format is also pretty lame: we just wrap the body of the each
help command in a code block (markdown's equivalent of <pre>).

R=brettw@chromium.org
BUG=468851

Review URL: https://codereview.chromium.org/1024783002

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Cr-Mirrored-Commit: e7700dd26912bcab168b6f4795a05fd9b5b1cb22