adding spectacle editor to list of extensions

docs/extensions.md

@@ -1,4 +1,5 @@
 <a name="third-party"></a>
-# Third Party Extensions
+# Spectacle Extensions
 
-- [Spectacle Code Slide](https://github.com/thejameskyle/spectacle-code-slide) - Step through lines of code using this awesome slide extension by @thejameskyle
+- [Spectacle Editor](https://www.formidable.com/open-source/spectacle-editor) - A desktop application to create your own Spectacle-based presentations using drag and drop, text editors, and more. Plotly integrated and open sourced. Created by [Formidable](https://www.formidable.com) in partnership with [Plotly](https://plot.ly).
+- [Spectacle Code Slide](https://github.com/thejameskyle/spectacle-code-slide) - Step through lines of code using this awesome slide extension by [@thejameskyle](https://github.com/thejameskyle).