✨ Make fadeIn prop in Img component accept number

Author: frinyvonnick

packages/gatsby-image/README.md

@@ -337,7 +337,7 @@ You will need to add it in your graphql query as is shown in the following snipp
 | ---------------------- | ------------------- | --------------------------------------------------------------------------------------------------------------------------- |
 | `fixed`                | `object`            | Data returned from the `fixed` query                                                                                        |
 | `fluid`                | `object`            | Data returned from the `fluid` query                                                                                        |
-| `fadeIn`               | `bool`              | Defaults to fading in the image on load                                                                                     |
+| `fadeIn`               | `number`            | Defaults to fading in the image on load in 500ms                                                                            |
 | `title`                | `string`            | Passed to the `img` element                                                                                                 |
 | `alt`                  | `string`            | Passed to the `img` element                                                                                                 |
 | `crossOrigin`          | `string`            | Passed to the `img` element                                                                                                 |

packages/gatsby-image/src/index.js

@@ -260,9 +260,13 @@ class Image extends React.Component {
       itemProp,
     } = convertProps(this.props)
 
-    const shouldReveal = this.state.imgLoaded || this.state.fadeIn === false
-    const shouldFadeIn = this.state.fadeIn === true && !this.state.imgCached
-    const durationFadeIn = `0.5s`
+    const shouldReveal =
+      this.state.imgLoaded || Boolean(this.state.fadeIn) === false
+    const shouldFadeIn =
+      Boolean(this.state.fadeIn) === true && !this.state.imgCached
+    const durationFadeIn = Number.isInteger(this.state.fadeIn)
+      ? `${this.state.fadeIn}ms`
+      : `0.5s`
 
     const imageStyle = {
       opacity: shouldReveal ? 1 : 0,