Provide sync API

[jverce]

Since it's a straightforward change (as far as I can tell), I'd like to work on providing an alternative synchronous API for this lib.

This would mean keeping the async API as-is:

• transform
• toJson
• prettyPrint

The new API would provide the new sync functions:

• transformSync
• toJsonSync
• prettyPrintSync

My proposal will most probably involve refactoring the async calls so that they return a Promise that wraps up a call to their corresponding sync alternative (since that's what's done currently). This change should be backwards compatible.

[tuananh]

I've been meaning to do that but the priority shifted. my current work has no need for xml transformation anymore.

If you want to work on that, you're more than welcome :)

[tuananh]

also: toJson is not yet implemented :D, i shouldn't have put it there since it's not done