Add handling of multiple header levels
remove trailing whitespace from sample output
always strip trailing whitespace from JSON
add logic to filter out empty markdown cells and strip excess whitespace
add logic to merge markdown blocks between horizontal rules
add CLI help for --join-blocks option
Add a README notebook generated from README.md
strip trailing whitespace from code cells
make joining blocks into single cells the default
reflect changes in README.ipynb
add notebook -> markdown conversion by shelling out to nbconvert