Widget for Unsplash integration

**Is your feature request related to a problem? Please describe.**
I have moved to SSG(Gatsby) from Ghost.org and Medium.com and one of there extremely nice feature with authoring was the idea of linking images from Unsplash. I believe this is one of the key features that a blog developer look at, especially because Unsplash provides free to use(with some attribution) content for the blogs/sites. I would usually use them for Cover/Featured image and sometimes in the main content as well. It is extremely irritating and time consuming process to find an image which can be reused in our site without much copyright issues. Finding/Downloading and then Uploading to our repo is a lengthy and irritating process.

**Describe the solution you'd like**
The way it worked in Ghost.org and Medium.com is pretty awesome. As a content writer we have an option to simply click a button that opens up a widget for exploring an image from thousands of Unsplash images.

A widget for Unsplash image can be really helpful that allows the writer to explore the images out of the box from the CMS.

I would expect it to add the public URL and attribution text to the markdown frontmatter, where it can be picked by the site.

**Describe alternatives you've considered**
The alternatives currently I am exploring is to manually go to Unsplash website and explore images. I would then look for the public URL and add it to a String URL in the content. Similarly, I would also look for attribution link and add it as text for the frontmatter.

**Additional context**
[Gatsby Integration Blog](https://medium.com/unsplash/a-few-of-unsplashs-api-partners-95e4bdc79870)

No files, tests, or entry points are named. Start by reviewing the CMS authoring experience and how image URLs and frontmatter are currently handled, then define the Unsplash search widget and attribution flow. Done means writers can explore Unsplash images and add the selected public URL and attribution text to frontmatter.

Written by the indexing model from the issue text.