templates: TLSVersion (#2323)

* new template action: TLS protocol version

* new template action: use caddytls.GetSupportedProtocolName

Avoids code duplication by reusing existing method to get TLS protocol
version used on connection. Also adds tests

caddyhttp/httpserver/tplcontext.go

@@ -31,6 +31,7 @@ import (
 
 	"os"
 
+	"github.com/mholt/caddy/caddytls"
 	"github.com/russross/blackfriday"
 )
 
@@ -448,6 +449,15 @@ func (c Context) AddLink(link string) string {
 	return ""
 }
 
+// Returns either TLS protocol version if TLS used or empty string otherwise
+func (c Context) TLSVersion() (ret string) {
+	if c.Req.TLS != nil {
+		// Safe to ignore an error
+		ret, _ = caddytls.GetSupportedProtocolName(c.Req.TLS.Version)
+	}
+	return
+}
+
 // buffer pool for .Include context actions
 var includeBufs = sync.Pool{
 	New: func() interface{} {