Fix inconsistent ipfs stats bw formatting

[mateon1]

The current way the command formats its output is inconsistent, sometimes the output is shorter and sometimes it's longer, which causes issues when using ipfs stats bw --poll
This PR makes the output always the same length.

Example of how it looks:

mateon1@deimos:~$ ipfs stats bw --poll -i 200ms
Total Up    Total Down  Rate Up     Rate Down
  2.1 GB      3.6 GB      3.4 kB/s    4.2 kB/s    

[whyrusleeping]

LGTM, this was bothering me the other day too. Thanks!