Add overlay_filter param to hero headers

The `overlay_filter` param lets you darken or otherwise filter the hero header picture to make the text content pop out more.

You can use it by specifying the opacity (between 0 and 1) of a black overlay like so:

```yaml
excerpt: "This post should display a **header with an overlay image**, if the theme supports it."
header:
  overlay_image: unsplash-image-1.jpg
  overlay_filter: 0.5 # same as adding an opacity of 0.5 to a black background
  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
  cta_label: "More Info"
  cta_url: "https://unsplash.com"
```

Or if you want to do more fancy things, go full rgba: 

```yaml
excerpt: "This post should display a **header with an overlay image**, if the theme supports it."
header:
  overlay_image: unsplash-image-1.jpg
  overlay_filter: rgba(255, 0, 0, 0.5)
  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
  cta_label: "More Info"
  cta_url: "https://unsplash.com"
```

Merge branch 'release/3.1.3'

Update CHANGELOG notes to version 3.1.3

Merge branch 'feature/artokai-patch-1' into develop

Merge branch 'patch-1' of https://github.com/artokai/minimal-mistakes into feature/artokai-patch-1

Fix SEO author bug
- If `twitter.username` is set and `author.twitter` is nil bad things happen.
- Close #289

Allow `author_profile` to be toggled in a _layout (eg. archive-taxonomy.html)
- Close #285

Remove white space as it was causing issues with Markdown and parsing as code blocks
- Close #279

Fix "build:all" npm script

The script "build:all" was missing the "npm run" section for the "build:js" part.

Fix site.og_image URL by appending /images/ the path
- Close #277

Update README

Update README

Merge branch 'release/3.1.2' into develop

Merge branch 'release/3.1.2'

Update CHANGELOG

Improve nav_list helper to be used with any type of navigation list defined in navigation.yml data file
- Close #272

Merge branch 'feature/related-posts-alignment' into develop

Decrease left/right page padding on small screens to give more space for content.

Fix related posts alignment
- Close #273

Fix alignment of "Follow" button in author profile
- Close #274