Add initial Markdown support

[strzibny]

This PR adds the basic support for our new Markdown Output feature, namely by introducing getMd and getMdBySearchId.

[catherine-serpapi]

Are these changes related to supporting Markdown?

[hilmanski]

It's just replacing the hard-coded version in deno.json. I believe it has nothing to do with the Markdown feature itself.

https://github.com/denoland/dnt

[hilmanski]

Thanks, @strzibny for the work here. I've tested it from a user perspective.

The getMd works great:

• Tested on an API outside Google; it also works well.

Test getMdBySearchID also works well:

For an async sample in Markdown in the README file:

• maybe we should use getMd({..}) instead on getJSON as that works as well.

I also want to suggest adding early on in the README that we support both JSON and Markdown.

[hilmanski]

Maybe, @zyc9012 or @tanysheng can have a final review for code-wise before merging.

[strzibny]

Good point, I separated Deno to a new PR #49 and I'll rebase this afterwards.

[strzibny]

@hilmanski I rebased since #49 is now merged and updated the getMdBySearchId example to get ID from getMd, good catch. @tanysheng could you do a final review for this?