Add command line option to ignore blank yaml values

[jsh2134]

It would be great if there was an output flag to ignore blank yaml values. Currently it outputs null

My use case is that I'm using it along with envdir to output a docker-compose file to an envdir directory

the current command:
cat docker-compose.yml | shyaml get-value api.environment | grep -v null | awk -F': ' '{print $2 > ("envdir/" $1)}'

proposed fix would be something like
shyaml get-value api.environment --ignore-blank

via a StackOverflow comment http://stackoverflow.com/questions/5014632/how-can-i-parse-a-yaml-file-from-a-linux-shell-script/15113891?noredirect=1#comment48491233_15113891

[vaab]

Hi, thanks for your suggestion. Actually, it's probably better to output 'null' only on request. I'm using postgres often, and you can actually set how null value should be outputted and it was usefull for me several times.

As a consequence, I would probably change shyaml to output an empty string for null values, and add a special --display-null-as VALUE options. I'm pretty much busy these days so don't expect an implementation of this right now, and I probably need to think it through a little more.