replace some basic uses of fmt.Sprintf()

Really tiny gains here, and doesn't significantly impact readability:

    BenchmarkSprintf
    BenchmarkSprintf-10    11528700     91.59 ns/op   32 B/op  1 allocs/op
    BenchmarkConcat
    BenchmarkConcat-10    100000000     11.76 ns/op    0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

plugin/plugin.go

@@ -130,7 +130,7 @@ func (r *Registration) Init(ic *InitContext) *Plugin {
 
 // URI returns the full plugin URI
 func (r *Registration) URI() string {
-	return fmt.Sprintf("%s.%s", r.Type, r.ID)
+	return r.Type.String() + "." + r.ID
 }
 
 var register = struct {