rename ypick back to ymask

The program doesn't pick values out as documents (though that might be a
good feature for yq), it masks a document and only prints the parts that
match.

Fixes #54
trim leading whitespace from CSV cells by default

Some CSV files have leading whitespace to make the columns line up for
easy visual inspection. By default, we want to remove that whitespace.

Also add a --no-trim option to turn this behavior off, if there's really
a need for it.

Fixes #49