Refactor reading from different sources #23
Description
Currently reading internal STT file and .ti files is configured in .sttrc and when multiple files are given, all of them are read one after the other.
What I want is not reading of all files at once but the possibility to read one file and convert it to the internal STT format or just reading it and displaying reports of the contents.
Idea: provide converting functionality, so the user can do stuff like
stt convert ~/.ti-sheet - | stt report --input-file -
where convert takes two files (or stdin/stdout when given the minus sign)
Additionally this enables the user to import/merge contents like
(stt convert ~/.ti-sheet -; cat ~/.stt) > ~/.stt_merged