Support custom PASSWORD_STORE_DIR setting

`pass` supports a user defined location for password storage by simply setting `PASSWORD_STORE_DIR`.  This patch makes `passmenu` support people like me who keep their pass data in a directory to fit the [XDG basedir](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html) ;)

Love the `passmenu` idea BTW, so obvious and yet never occurred to me at all.

Thanks,

James