Convenience method for writing FrontMatter files

[jbmorley]

It's currently necessary to write a few lines of code to correctly create a FrontMatter file. For example,

with open(path, "w") as fh:
    document = utils.FrontmatterDocument(metadata=metadata)
    fh.write(frontmatter.dumps(document))
    fh.write("\n")

Since this is something we might expect site-specific plugins to do fairly frequently, it would be helpful to create a convenience method for exactly this purpose.