Add Plotly detection

This can be tested [here](https://plot.ly/javascript/)

Thanks to @gadcam for the review.

src/apps.json

@@ -5665,6 +5665,16 @@
 			},
 			"website": "plone.org"
 		},
+		"Plotly": {
+			"cats": [
+				25
+			],
+			"icon": "Plotly.png",
+			"implies": "D3",
+			"env": "Plotly",
+			"script": "https?://cdn\\.plot\\.ly/plotly",
+			"website": "plot.ly/javascript/"
+		},
 		"Plura": {
 			"cats": [
 				19