Multicore support

[krlmlr]

for style_files(). Could be on by default, works for me except for the output of incomplete lines (to indicate progress). I think we can solve this by printing complete lines after completion in the multicore case.

We need to evaluate if it's worth to enable multiprocess on Windows.

[lorenzwalthert]

Refrence: #277.
Also, note that purrr::map() is going to support multicores soon(ish). tidyverse/purrr#121 (comment)

[krlmlr]

Oh, I searched for "parallel" and "multicore"...