Make excerpts optional in header overlay
Some posts and some header images don't lend themselves well to
displaying the excerpt in the overlay. Make this optional by
introducting a new boolean variable:
`page.header.show_overlay_excerpt`
Set it to implicitly default to `true` so existing users are unaffected by
testing explicitly for "!= false".
This can be enabled globally for a site by changing the default to `false`
in the local _config.yml, or per page by specifying the value in front
matter.
Document show_overlay_excerpt variable
* Add show_overlay_excerpt variable to the Header Overlay field documentation table.
Close#1429
* Added support for an alt tag for the header image
This allows using page.header.image_description as the alt tag. It
will still default to site.title if unset.
* Added documentation
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"
```
