This fixes#20 by adding support for TOML-based metadata for symbols.
There's now a template TOML file in `svg/_template.toml` that documents all the different metadata properties. The file can be copied to match the name of an SVG file (but with a `.toml` extension) and it will be processed by the vocabulary builder when the site is generated.
Currently there are only two properties documented, `always_nest` and `always_be_nested` (as per the needs outlined in https://github.com/toolness/mystic-symbolic/issues/17#issuecomment-786696966), but they don't actually do anything yet (actual support for _using_ the metadata will come in another file).
Right now the TOML files are validated quite stringently: if a file contains the name of a property it doesn't understand, or the type of the property is wrong, it will raise an error. We can revisit this if it becomes burdensome.
Nina Paley