Take a map and return a Frontmatter struct, taking FrontmatterMapping into account.

Convert a frontmatter_mapping (interface{} due to how Viper works) to a FrontmatterMapping struct.

Load a slice of Destinations and Sources from values passed which the calling module shoukd have read from config If selectedDestinations is empty or nil, all destinations are loaded.

This is more just a set of defaults compatible with Hugo's frontmatter.