Make styled-jsx optional.

[arunoda]

This fixes #964
This is to support plugins like styled-jsx-postcss

But I'd like to solve this from styled-jsx end. styled-jsx should parse the content only once.
Even it's included multiple times.

Since this works earlier, I think something got changed recently related to this in the styled-jsx side.
cc @rauchg @nkzawa @giuseppeg

[rauchg]

The problem with this option is that, if styled-jsx is not substituted the code will fail? Since we import the module for server rendering anyways ?

[arunoda]

@rauchg other option is to make styled-jsx idempotent. Even it's added multiple times, it should run only once.

When I was reading the code, it seems like it's doing like that for <style> tags. But may be need to do it for adding the styled-jsx/style import.

[rauchg]

@rauchg other option is to make styled-jsx idempotent. Even it's added multiple times, it should run only once.

Yes please

[arunoda]

@rauchg I think we need to discuss this over in styled-jsx. I hope this task will be pretty simple for someone who knows about the styled-jsx code base.

[timneutkens]

@arunoda vercel/styled-jsx#122