Merge branch 'release/4.0.4'

Conflicts:
	README.md

Merge branch 'feature/theme-gem' into develop

Remove `prepend: "/"` from includes and add to image path's instead

Replace base_path with `absolute_url` filter where possible

Allow images to be placed in other folders
- Remove `images/` only restriction and encourage placement in `assets/images/` instead

Merge branch 'release/3.3.5'

Add `en` default text to `site.locale` strings
- Close #407

Merge branch 'release/3.2.3'

Replace contains "http" with contains "://"

Merge branch 'release/3.1.4'

Merge branch 'feature/overlay-filter' into develop

Merge branch 'tobie-hero-filter' of https://github.com/tobie/minimal-mistakes into feature/overlay-filter

Style fixes

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/3.0.0'

Merge branch 'feature/documentation-and-demo' into develop

Make base_path available to all includes that need it

If page doesn't have a title fallback to site.title

Improve URL handling for internally and externally linked pages

Reduce button size