IntInSlice and StringInSlice functions

Refs https://github.com/tendermint/tendermint/pull/835

common/string_test.go

@@ -0,0 +1,14 @@
+package common
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStringInSlice(t *testing.T) {
+	assert.True(t, StringInSlice("a", []string{"a", "b", "c"}))
+	assert.False(t, StringInSlice("d", []string{"a", "b", "c"}))
+	assert.True(t, StringInSlice("", []string{""}))
+	assert.False(t, StringInSlice("", []string{}))
+}