Add new method regexMatchGroup

[suikast42]

For example. To extract traefik hostnames with consul-tepmplate a template like shown below could be very handy.

This template could be very helpful for dns resolution on some limited client networks for example.

{{ range services }}
  {{ range .Tags }}
    {{ $pattern := `traefik\.http\.routers\..+\.rule=Host\('([a-zA-Z0-9.-]+)'\)` }}
    {{ $matches := regexMatchGroup $pattern . }}
    {{ if $matches }}
      {{ $extractedDomain := index (index $matches 0) 1 }}
      Extracted Domain: {{ $extractedDomain }}
    {{ end }}
  {{ end }}
{{ end }}