New rule: modifies-parameter (#25)

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

* Fix superfluous-else rule struct namming.

* Adds superfuous-else rule to the rules table

* Adds confusing-naming rule

* adds multifile test

* clean-up

* fix config.go

* working version

test/modifies-param_test.go

@@ -0,0 +1,12 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mgechev/revive/rule"
+)
+
+// TestModifiesParam rule.
+func TestModifiesParam(t *testing.T) {
+	testRule(t, "modifies-param", &rule.ModifiesParamRule{})
+}