* add og:locale meta tag
* improve meta tags:
- fix `author` and `og:url` meta tags
- add missing (Facebook) Open Graph meta tags
- add `og:locale`
- add `og:updated_time`
- make `og:type` an `article` for blog posts (instead of `website`)
- add `article:` meta tags for blog posts (use the first category of blog posts for `article:section`)
- add `og:image:type` MIME type
- add auto-calculated `og:image:width` and `og:image:height`
- you can now set the key `facebook_author = "xyz"` in the front matter of posts (also possible to set a fallback under `Site.Params.facebook_author`) to have the `article:author` meta tag set to `"https://www.facebook.com/xyz/"`
- add `article:publisher` if `facebook_site` is set in `Site.Params`
- use the post banner image for both `og:image` and `twitter:image` if present and fall back to `static/img/sharing-default.png` otherwise
- make `twitter:card` only of type `summary_large_image` if page is a blog post and has a dedicated banner image set
- add `twitter:creator` meta tag if the key `twitter_author` is present in the front matter of posts (also possible to set a fallback under `Site.Params.twitter_author`)
- added some examples for facebook/twitter authors in the example `.md` posts
- make `twitter:site` meta tag disappear if `twitter_site` is not set in `Site.Params`
- truncate `twitter:title` and `twitter:description` to max allowed length (70 and 200 chars)
- `markdownify` and `plainify` various strings for convenience
- simplify code/improve style
* use additional variables to avoid code redundancy
* document how to set HTML metadata
* clarify metadata documentation
* also mention `og:updated_time`
* clarify sentence about HTML metadata
