Simplified ESM import.

The issue #303 was opened because twemoji library cannot be
imported through modern browsers as ECMAScript module.

This commit adds the simplest way to make any generic namespace
exported as default (esm.sh) and the raw copy of the minified library
exported as default `twemoji` module.

esm.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+if [ ! -f "$1" ] || [ "$2" = "" ]; then
+  echo '
+usage example:
+  ./esm.sh ./path/module.js modname
+
+will append:
+  export default modname;
+
+at the end of:
+  ./path/esm.js
+'
+else
+  cp "$1" "$(dirname "$1")/esm.js"
+echo "
+export default $2;
+" >> "$(dirname "$1")/esm.js"
+echo "
+exported $2 as default
+"
+fi