Added an ability to customize contents of meta description tag in
templates that inherit from layout. This replaces the default
description previously used in all views.
This change could help with SEO and missing descriptions in search
results (#167). It's based on advice from
https://developers.google.com/search/docs/appearance/snippet
> Create unique descriptions for each page on your site
> Programmatically generate descriptions
This is not guaranteed to fix the issue, search engines are opaque.
All custom descriptions start with "Elixir Cross Referencer - ".
Currently customized descriptions:
* source - path to the file, project name and version
* ident - name of the searched identifier, project name and version
Everything else uses the previous, default description.
