Add support for XDG base directory config

[omroux]

Why

Add support for XDG base directory config to stop polluting home directory using github.com/adrg/xdg library while keeping support for legacy ~/.pomodoro.

This fixes #32

Tested With

Starting with no config directory at all I initialized ~/.pomodoro and used my version to test backwards compatibility:

$ rm -rf ~/.pomodoro/ ~/.config/pomodoro/
$ pomodoro start
24:59⏱  1🍅
$ ls ~/.pomodoro/
current  history
$ go run . status
24:50⏱  1🍅
$ ls ~/.pomodoro/
current  history
$ ls ~/.config/pomodoro/
ls: cannot access '/home/omroux/.config/pomodoro/': No such file or directory

And when using just my version:

$ rm -rf ~/.pomodoro/ ~/.config/pomodoro/
$ go run . start
$ ls ~/.config/pomodoro/
current  history
$ ls ~/.pomodoro/
ls: cannot access '/home/omroux/.pomodoro/': No such file or directory

Making sure --directory still works:

$ go run . --directory pomodoro start
24:59⏱  1🍅
$ ls pomodoro/
current  history