Add ES module support to leverage code splitting and tree shaking

The purpose of this PR is to add an option to export the ejs template as an ES6 Module in order to leverage Code Splitting and Tree Shaking. This is pretty important to a lot of people who are still using legacy ejs templates, but want to optimize their bundles.

There were a few things I had to change to get this to working:

1) Why is variable required: When lodash or underscore templates are instantiated without a variable option, a with statement is used. This violates strict mode and fails compilation.

2) Why not use a default? What makes using a default difficult in this case is the template needs to know the variables being passed into it. In order to likely do something like this, we would have to change interpolation settings. This seems to be a lot of effort to go through to support a default case. We could set a default and not handle the context, but my fear would be that the template compiles find at build, but fails at runtime, which could be risky.

2) Not being able to leverage requireFromString. Since requireFromString only supports for module.exports, I had to get creative with parsing export default for ES Modules. Since I could not find a solution that already handles this, I wrote a RegExp that removes the string and creates a new function/

Please see #41 for commit 814bcc5. And please let me know if there is anything you all would like to see improved or changed :smile:

difelice / ejs-loader

Add ES module support to leverage code splitting and tree shaking #42