Configure `--missing-key-action` on a per-variable basis.

[josh-padnick]

Boilerplate currently supports the --missing-key-action property when using boilerplate on the command line.

The problem is that I want certain values to render blank, while other values that are accidentally blank (or otherwise invalid) to throw an error. But boilerplate makes me choose just one --missing-key-action approach for the entire template generation.