fix(output): preserve directory structure in output (fix issue #13)

[smhagit]

Introduce a new parameter to preserve the output directory structure (if the input is a directory).
The new preserveStructure parameter defaults to false to don't break any existing CI/CD scripts which rely on the previous (put all in one directory) behaviour.

I bumped the version from 2.2.0 to 2.3.0.

This resolves issue #13

[runegan]

Great job! Thank you! I haven't been able to test the new functionality, but it seems to be backwards compatible at least.

Could you write one or two tests for it in "test.js", where it tests if it creates the desired output with an input? And testing it with "npm test"

I'm also trying to keep the code uniform, on smhagit-preserve-structure I've run eslint to apply the styling. So it would be great if you could merge with that.

Hope that is doable.

Thank you for the pull request!