WIP: generate API docs

[sqs]

fix #46

I want this only to emit sourcegraph.d.ts API docs, not internal API docs, but I haven't figured out how to do that yet.

[cspotcode]

Can you use @internal annotations plus tsc's stripInternal option?

[sqs]

I will try that. Thanks!

[chrismwendt]

Here's how to move this PR to https://github.com/sourcegraph/sourcegraph/tree/master/packages/sourcegraph-extension-api

cd browser-extensions
git format-patch master --stdout > /tmp/patch

cd ../sourcegraph
cat /tmp/patch | git am -3 --directory=packages/sourcegraph-extension-api/
# and fixup merge conflicts