Polish OUTPUT_FILE_EXTENSION

[lpiwowar]

Is there any reason why this should be anything else than txt? If there is, it should be a parameter, if not, it shouldn't be a constant.

Originally posted by @jpodivin in #71 (comment)

[Akrog]

I think it should be a constant, and it should be .md, that's why the code added that global value, to prepare for when we make the change.