accept CSS string in registerPreviewStyle

[erquhart]

- Summary

Raw styles can't currently be injected into the preview pane, which is a challenge for projects using modern build tooling. For example, I'd like to import styles from a specific file using inline webpack loaders and pass the result to registerPreviewStyle, rather than doing something hacky to get Webpack to emit files, which is next to impossible if the webpack config is shared, as in systems like Gatsby.

Adding this as a beta feature only to avoid commitment to the exact API being introduced.

- Description for the changelog

Accept CSS strings in registerPreviewStyle

- Docs

Raw CSS in registerPreviewStyle

registerPreviewStyle can now accept a CSS string, in addition to accepting a url. The feature is activated by passing in an object as the second argument, with raw set to a truthy value.This is critical for integrating with modern build tooling. Here's an example using webpack:

/**
 * Assumes a webpack project with `sass-loader` and `css-loader` installed.
 * Takes advantage of the `toString` method in the return value of `css-loader`.
 */
import CMS from 'netlify-cms';
import styles from '!css-loader!sass-loader!../main.scss'
 
CMS.registerPreviewStyle(styles.toString(), { raw: true })

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit a31d48e

https://deploy-preview-1162--netlify-cms-www.netlify.com

[verythorough]

Deploy preview for cms-demo ready!

Built with commit fa23226

https://deploy-preview-1162--cms-demo.netlify.com

[verythorough]

Deploy preview for cms-demo ready!

Built with commit a31d48e

https://deploy-preview-1162--cms-demo.netlify.com

[tech4him1]

LGTM