Added a --pad-prefix flag which makes all prefixes the same length, padding shorter ones with spaces.

Example:

$ concurrently -n foo,barbaz --pad-prefix "echo foo" "echo bar"
[foo   ] foo
[foo   ] echo foo exited with code 0
[barbaz] bar
[barbaz] echo bar exited with code 0

Noteworthy:
Although most prefixes have a stable length, PIDs can become longer if the command restarts, e.g. it was originally 9 but after a restart it's 10.
Obviously we can't rewrite the past, so new log entries from that respawn onwards will be padded appropriately.

Closes #166
Closes #417