Add support to export as namespace

[devaradhan]

Thanks for the awesome plugin.
It will be cool if we also have support for typings exported as namespace as well.
We are creating a bundle with var as library target using webpack, allowing users to use symbol from global namespace directly, without importing. We want to generate d.ts file for this as well.

[tonym97]

FYI I just added this:
if (outputAsModuleFolder) {
content += '\nexport as namespace ' + exportName + ';';
}

as a lazy man's way of doing this. Will create a pull request when I can