Pass metadata via command line

Use-case: I have some common metadata that I want to pass to all documents in a set, but don't want to (or can't) add it to every content file.

Proposed solution: Allow metadata key/value pairs as command-line arguments to `multimarkdown`, so the common metadata can be mixed with the metadata (if any) already in the file.

Aside from supplementing existing metadata, this could also be useful for overriding the value of metadata keys already in the file.