Merge branch 'release/3.1.4'

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"
```

CHANGELOG.md

@@ -1,3 +1,13 @@
+## [3.1.4](https://github.com/mmistakes/minimal-mistakes/releases/tag/3.1.4)
+
+### Enhancements
+
+- Add overlay_filter param to hero headers [#298](https://github.com/mmistakes/minimal-mistakes/pull/298)
+
+### Maintenance
+
+- Fix `www` and `https` links in author profile include [#293](https://github.com/mmistakes/minimal-mistakes/pull/293)
+
 ## [3.1.3](https://github.com/mmistakes/minimal-mistakes/releases/tag/3.1.3)
 
 ### Enhancements

_includes/page__hero.html

@@ -18,8 +18,14 @@
   {% capture overlay_img_path %}{{ page.header.overlay_image | prepend: "/images/" | prepend: base_path }}{% endcapture %}
 {% endif %}
 
+{% if page.header.overlay_filter contains "rgba" %}
+  {% capture overlay_filter %}{{ page.header.overlay_filter }}{% endcapture %}
+{% elsif page.header.overlay_filter %}
+  {% capture overlay_filter %}rgba(0, 0, 0, {{ page.header.overlay_filter }}){% endcapture %}
+{% endif %}
+
 <div class="page__hero{% if page.header.overlay_color or page.header.overlay_image %}--overlay{% endif %}"
-  style="{% if page.header.overlay_color %}background-color: {{ page.header.overlay_color | default: 'transparent' }};{% endif %} {% if overlay_img_path %}background-image: url('{{ overlay_img_path }}');{% endif %}"
+  style="{% if page.header.overlay_color %}background-color: {{ page.header.overlay_color | default: 'transparent' }};{% endif %} {% if overlay_img_path %}background-image: {% if overlay_filter %}linear-gradient({{ overlay_filter }}, {{ overlay_filter }}), {% endif %}url('{{ overlay_img_path }}');{% endif %}"
 >
   {% if page.header.overlay_color or page.header.overlay_image %}
     <div class="wrapper">
@@ -46,4 +52,4 @@
   {% if page.header.caption %}
     <span class="page__hero-caption">{{ page.header.caption | markdownify | remove: "<p>" | remove: "</p>" }}</span>
   {% endif %}
-</div>
+</div>