Option --coalesce-streams should work with --execute and all output formats

[knuesel]

Currently the following doesn't work:

jupyter nbconvert --execute --to html --coalesce-streams file.ipynb

It seems two things are preventing this to work:

1. The option --coalesce-streams seems to have no effect when --execute is used
2. The option --coalesce-streams forces the output to notebook format (this is documented)

It would be nice to lift these restrictions, as currently this job requires three separate steps:

jupyter nbconvert --execute --inplace file.ipynb
jupyter nbconvert --coalesce-streams --inplace file.ipynb
jupyter nbconvert --to html file.ipynb