This patch adds Arch/pacman style output when invoking 'guix download'.

Previous output:
$ guix download ftp://ftp.gnu.org/gnu/guile/guile-2.0.11.tar.gz
starting download of `/tmp/guix-file.IgqbP2' from `
ftp://ftp.gnu.org/gnu/guile/guile-2.0.11.tar.gz'...
ftp://ftp.gnu.org/.../guile-2.0.11.tar.gz     29.6% of 7339.9 KiB (781.
KiB/s)

Patched output:
$ guix download ftp://ftp.gnu.org/gnu/guile/guile-2.0.11.tar.gz
starting download of `/tmp/guix-file.ULgbny' from `
ftp://ftp.gnu.org/gnu/guile/guile-2.0.11.tar.gz'...
 guile-2.0.11.tar.gz  7.2MiB               1.1MiB/s 00:00:02
[#######             ]  36.6%

-Steve