TextToRank should accept interface instead of ParsedSentence as struct

[koushki]

If I want to use convert.TextToRank and I have a customized rule, it is not possible to use convert package because it accepts parse.ParsedSentence struct as the first argument.

I had a quick look and it seems should be possible to change parse.ParsedSentence to an interface.

I can create a PR if it does make sense to you @DavidBelicza!