Add function for HMAC SHA256

[thevilledev]

Useful for cases where data in Consul has to be rendered into HMAC SHA256 hashes, where the shared key is stored in Vault. Example snippet:

{{ range ls "foo" }}
{{ $msg := .Value -}}
{{ with secret "secret/foo" }}{{ .Data.data.bar | hmacSHA256Hex $msg }}{{ end -}}
{{ end }}

Fixes #1701.