Twitter Cards: add twitter:image, twitter:title and twitter:description

Problem

layouts/partials/head.html:16-18 only declares twitter:card and twitter:site. Twitter falls back to the og:* tags, so it works — but only if the OG image exists (see #52), and it is more robust to declare the Twitter tags explicitly.

To do

• Add:

<meta name="twitter:title" content="{{ if .IsHome }}{{ .Site.Title }}{{ else }}{{ .Title }}{{ end }}">
<meta name="twitter:description" content="{{ with .Description }}{{ . }}{{ else }}{{ .Site.Params.description }}{{ end }}">
{{ with .Site.Params.ogImage }}<meta name="twitter:image" content="{{ . | absURL }}">{{ end }}

Depends on #52.

Start in layouts/partials/head.html at lines 16-18 and review issue #52 for the OG image dependency. Add the three explicit Twitter meta tags shown in the issue, then verify the generated page head includes the title, description, and image tags when an OG image is configured.