Refactor Oniguruma integration

Instead of use a command to change imports before build, using a build tag to generate the correct binary.

This will allow applications to compile enry using oniguruma with less troubles.

Signed-off-by: Antonio Jesus Navarro Perez <antnavper@gmail.com>

regex/oniguruma.go

@@ -0,0 +1,17 @@
+// +build oniguruma
+
+package regex
+
+import (
+	"github.com/moovweb/rubex"
+)
+
+type EnryRegexp = *rubex.Regexp
+
+func MustCompile(str string) EnryRegexp {
+	return rubex.MustCompile(str)
+}
+
+func QuoteMeta(s string) string {
+	return rubex.QuoteMeta(s)
+}