pretty print of resultant kml/xml data

[birdage]

right now the KML data comes out completely unformatted, it would be good to have a pretty print option for the resultant KML string.

[birdage]

could be as easy as adding in https://github.com/vkiryukhin/pretty-data, and an option flag. Happy to make the PR if wanted....?

[haoliangyu]

https://www.npmjs.com/package/gtran-kml is a wrapper of the tokml library and I have added the pretty option to prettify output.