backend/mappings: Split mappings out into several files
backend/mappings: Add a mapping for Telugu
backend/converters: Port to split mappings and add support for Telugu
backend: Many changes, detailed below

* Rename split mappings files to ISO language code names
* Re-factor converters.py and add language auto-detect for input
* Bengali nukta characters are composed from multiple unicode chars
* Fix some minor typos with mappings files
* Port tests to all these changes
* Add new tests for autodetection, and stub tests for new languages