multiline support

[vors]

• Multiline code cell hangs the kernel
• Figure out how to handle incomplete cell, i.e. @(

[vors]

The issue is caused by https://stackoverflow.com/questions/37417613/powershell-fails-to-run-multi-line-commands-from-stdin/37418731

[vors]

I took first stab at it in 45c0869 , now we don't hang on completed multi-line commands.

• we hang on incompleted commands
• we need to strip out the duplicated stdin from stdout that for some reason powershell sends there too (maybe because we run with -File -?)