Fix for tab indent

[itchyny]

If tab indentation exists, the command GhcModType does not work properly.

This pull request fixes this problem.

Test code. (GitHub converts the tab indent to four spaces, so please substitute by the command %s/^\s\+/\t/g)

main :: IO ()
main = do
    putStrLn "hello"
    putStrLn "world"
    putStrLn      $         "hello" ++ " world"
    putStrLn    (      "world"        ++    " hello"  )

[eagletmt]

You should not use hard tabs in Haskell IMO. Thanks though.

[itchyny]

I totally agree with that! Otherwise I would have found this bug much earlier. Thanks for merging.