Add conditional omission of fields based on their values

[squidfingers]

I would like a way to prevent a param from be written to the frontmatter, if it has a default value or it's false, just to keep my frontmatter as clean as possible. Maybe something like omit_if_default or omit_if_false options.

For example, only render draft:true if the value is true, otherwise, don't render the param.