support async call on windows

[Hi-Alan]

While a long time running task be dispatched with a flag such as #async on windows, shell2http return immediately.

set chrome="C:\Progra~2\Google\Chrome\Application\chrome.exe"
D:\bin\shell2http\shell2http -host=127.0.0.1 -port=8081 -no-index -log=access.log -form ^
  /html2pdf#async  "echo OK && %chrome% --headless --disable-gpu --print-to-pdf-no-header --print-to-pdf=%%v_file%% %%v_url%%"

[msoap]

I think, it's a duplicate of #70

[Hi-Alan]

Thanks for your works, msoap.

There is no command like nohup on windows. Requests will be blocked like this:

shell2http -port=8081 /longrun "start /b ping -n 5 127.0.0.1 >> D:\x.log"
shell2http -port=8081 /longrun "cmd /c ping -n 5 127.0.0.1 >> D:\x.log"