Add initial Markdown support

[strzibny]

This is WIP change for Markdown support.

[strzibny]

Maybe we should call this .md() instead of .markdown() to align with serpapi/serpapi-python#39

[trusche]

Why separate format and output arguments?

[trusche]

Maybe we should call this .md() instead of .markdown() to align with serpapi/serpapi-python#39

Personally I prefer markdown() over md()

[strzibny]

Thanks @trusche for having a look. You are right on search_archive, we should cleanly just pass the extension. I fixed it.

Personally I prefer markdown() over md()

Well, that's how I started as well :) However, consistency among our clients is more important I think (already in Python and PHP libs) and I think there is a tiny advantage of matching the extension.

[trusche]

Ruby and rails offer a lot of syntactic sugar, we could at least create an alias?

[strzibny]

I was thinking about it, but on the other hand I want us to be direct in docs and want to send people to one method everywhere. So creating an alias just on its own doesn't feel that good to me.

json, html, md are all shortcuts/abbreviations and in that sense it just works in uniform. I think that in the end uniformity among the abbreviations and across the libraries is just more important. They also directly suggest the extension in their meaning.

Do you think it's still worth it?

[pulkitchowdry]

@strzibny Thanks for the efforts. Functionality wise everything looks good to me 👍

One minor thing I observed (not due to code changes as part of this PR) is that when I pass output: "md" or output: "html" as a parameter in my SerpApi params then my output response for search method is broken with JSON parse related errors (since we default decoder to json. For any other values, for example output: "markdown" we throw an error instead of a broken response. Should we also throw an error for this? maybe we can check under a new issue.

I feel it would be good to mention some sample code in our Readme doc. Something like below where we are differentiating the three,

# For JSON output
result_json     = serpapi_client.search

# For HTML output
result_html     = serpapi_client.html

# For Markdown output
result_md       = serpapi_client.md

Note - I observe that our python library supports output in params so maybe in the future we can also support it in Ruby.

[pulkitchowdry]

LGTM 👍

[strzibny]

Thanks @pulkitchowdry, you are right, we should have already been choosing the right decoder for HTML. I included it in this change.