Combine output

[mark-00]

Add option combined_output to combine stdout and stderr in output

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[pablochacin]

@mark-00 thanks for the contribution. After #31 was merged, you need to rebase this PR.

Also, I think the description is not correct, as the PR maintains the normal behavior but adds a new option to return the combined output. Plase update it.

[mark-00]

@mark-00 thanks for the contribution. After #31 was merged, you need to rebase this PR.

Also, I think the description is not correct, as the PR maintains the normal behavior but adds a new option to return the combined output. Plase update it.

done

[pablochacin]

I did some minor suggestions to make the option's name consistent with the other options.

[mark-00]

I did some minor suggestions to make the option's name consistent with the other options.

I commited the suggestions