feat(templater): Added support for file include

Adds a 'fileContent' template function to insert the literal contents
of a file specified in the template.

Signed-off-by: Niklas Wik <niklas.wik@nokia.com>

templater/templater.go

@@ -193,6 +193,7 @@ func templateFuncs() template.FuncMap {
 	}
 	m["passLookup"] = GetFromPass
 	m["lookupIPAddr"] = GetIPsFromDNS
+	m["fileContent"] = GetFromFile
 
 	return m
 }