Add an option to allow manifest-path to be used

[cswindle]

Here is my updated PR which makes the manifest-path optional. I have also switched to output stdout/stderr, rather than hiding it.

[imp]

I see that you've dropped the output indentation. Was it on purpose ? If so, could you elaborate why please ?

[cswindle]

I did not purposely drop the indentation (as I did not realize that you had indentation), although the way that you had previously implemented it meant that it buffered all of the output before dumping stdout at the end, which means that for a long running cargo command there is no feedback on progress and it looks like multi has hung.