Support to custom file extension (*.sst -> *.ldb)

[dyorgio]

Hi guys,

It is a feature support: Include on options a fallback to file extension (relative with #119).
Current impl uses sst as fixed file extension, but Skype uses ldb...

Currently I need to rename all files in folder to get it working..

[dain]

This project is in maintenance mode, but that sounds pretty simple. Assuming the change is straight forward, I'll merge and release if you send a PR.