Respect XDG Base Dir Spec for config file

[alichtman]

Progress on #171. Note that this is not truly respecting the XDG Base Dir Spec, as the storage dirs now live in ~/.config. Works well enough for my purposes, though.

[kakawait]

I'm not maintainer at all, I just show that project and I'd like to use it.
But as you suggest, I'd like to have support of XDG base dir standard to avoid dotfiles of the hell at $HOME.

But I think you may support both, in order to do no create a big breaking changes.

Config should be able to check $HOME/.taskbook.json and fallback to file inside $XDG_CONFIG_HOME or $HOME/.config/

Furthermore, I think for storage.js we may continue using XDG base dir standard and put it at $XDG_DATA_DIR or $HOME/.local/share/. But same a previous we must find a way to do not break existing user.