Add a common regexp cache

```
BenchmarkGetOrCompileRegexp-10    	73959368	        13.71 ns/op	       0 B/op	       0 allocs/op
BenchmarkCompileRegexp-10         	 3143529	       380.1 ns/op	     872 B/op	      10 allocs/op
```

common/hstrings/strings_test.go

@@ -14,6 +14,7 @@
 package hstrings
 
 import (
+	"regexp"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -34,3 +35,24 @@ func TestStringEqualFold(t *testing.T) {
 	c.Assert(StringEqualFold(s1).Eq("b"), qt.Equals, false)
 
 }
+
+func TestGetOrCompileRegexp(t *testing.T) {
+	c := qt.New(t)
+
+	re, err := GetOrCompileRegexp(`\d+`)
+	c.Assert(err, qt.IsNil)
+	c.Assert(re.MatchString("123"), qt.Equals, true)
+
+}
+
+func BenchmarkGetOrCompileRegexp(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		GetOrCompileRegexp(`\d+`)
+	}
+}
+
+func BenchmarkCompileRegexp(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		regexp.MustCompile(`\d+`)
+	}
+}