Adds rule superfluous-else (an extension of indent-error-flow) (#13)

* Adds rule superfluous-else (an extension of indent-error-flow)

* Fix superfluous-else rule struct namming.

* Adds superfuous-else rule to the rules table

test/superfluous-else_test.go

@@ -0,0 +1,12 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mgechev/revive/rule"
+)
+
+// TestSuperfluousElse rule.
+func TestSuperfluousElse(t *testing.T) {
+	testRule(t, "superfluous-else", &rule.SuperfluousElseRule{})
+}