Restore sos convert --execute

[BoPeng]

We deprecated --execute option because our implementation was less flexible and powerful than papermill --engine sos. However, to convert a Rmd file to HTML, we will need to run the following commands do

sos convert file.Rmd file.ipynb
papermill --engine file.ipynb file_executed.ipynb
sos convert file_executed.ipynb file.html

It would be useful to condense these commands to

sos convert file.Rmd file.ipynb
sos convert file.ipynb file.html --execute

or even

sos convert file.Rmd file.html --execute