new preview handling has broken plugin functionality

[halvves]

Expected Behavior

Plugin authors should have control over the parent node of the preview pane.

Actual Behavior

This commit has broken my plugins' (halvves/zazu-giphy & halvves/zazu-colors) preview behavior (see: halvves/zazu-giphy#6). We may want to make the default DOM element on line 69 a simple div. Right now the <Markdown/> component is also stripping <style/> tags, rendering previewCss useless. We probably should include style in the allowList prop detailed here to fix.

Steps to reproduce the behavior

Install either halvves/zazu-giphy or halvves/zazu-colors and see that since moving from <Iframe/> to <Markdown/> rendered blockquote previews are now no longer functioning as expected.

Versions

System: MacOS 10.14.6
App Environment: production
App Version: 0.6.0
Electron Version: 5.0.4
Node Version: 12.0.0
Chrome Version: 73.0.3683.121

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I'm definitely willing to take a stab at fixing. I just wanted to open an issue and detail what I wanted to fix before I threw any time at it.