Escape interpolations by default

This is in order to get the expected handlebars-like mustache
escaping behavior. Unescaped interpolation made possible via the
common tripple curlies mustache.
I might have written this as
```js
return template
  .replace(/\{\{\{(.*?)\}\}\}/g, function(i, match) {
    return get(data, match);
  }).replace(/\{\{(.*?)\}\}/g, function(i, match) {
    return escapeExpression( get(data, match) );
  });
```