Feature/multifile

Multifile support; cleaned and finalised.

From the README: 

    You can pass the option `--file=` to point todotxt to another file. You
    can pass an alias, defined in the configuration, or the path to an
    arbitrary file.

    With a file wishlist, in the configuration defined as "wishlist", you
    can run:

        todotxt ls --file=wishlist

    To list all items form this wishlist file. Alternatively you can run:

        todotxt ls --file="~/Dropbox/todo/deferred.txt"

    To list all items from the file deferred.txt, provided that file
    exists.

    In order to list all items from all files defined in the config, use the
    `--all` flag with ls:

        todotxt ls --all