ttrss-passwordfile configuration option, ttrss user/login prompts

Add ttrss-passwordfile configuration option by copying googlereader-passwordfile implementation.

Add prompts for ttrss user/login when configuration file does not specify them by copying  googlereader implementation.

I'm quite a novice at this, so if this is poorly implemented or if you don't want this for any reason, feel free to disregard. If you'd like to add it but need changes to be made before committing, please let me know.