auto colspec option for fwf

[changhiskhan]

run through file and find the positions that are blank on every line
use the first position if there is a block

useful for small files and/or copy-pasting small DataFrame console output and then reconstructing via read_fwf

[sanand0]

+1 -- would be great to have this feature

[jreback]

see #4955, is in 0.13 (u can try 0.13rc1 now)
forgot to close this issue