Render src and alt attributes in content.image as Go templates

[traut]

At the moment, the attribute values are treated as plain strings which limits their usability:

    section "screenshots" {
      title = "Screenshots"

      content image {
        src = "{{ index (index .data.inline.findings.hosts 1).screenshots 0 }}"
      }
    }

produces

### Screenshots

![]({{ index (index .data.inline.findings.hosts 1).screenshots 0 }})