masthead: Prefer tagline over description and add defaults for title.

[olifre]

This is an enhancement or feature.

Summary

SEO usually prefers a slightly longer description, so jekyll-seo-tag allows to separate a short tagline from a longer description. This PR prefers site.tagline over site.description if it is set, and also adds another fallback to site.github.project_tagline. It also adds a fallback to site.github.repository_name in case site.title is not set.