Add Experimental Extensions

After seeing an issue on the Microformats repo about handling foreign objects, I'd like to introduce an conditional flag to enable "experimental" options and parsing, similar to what the PHP library mf2-cleaner aims to do.

• Provide microformats::extensions module.
• Add method for removing false positives (i.e.: unrecognized specified objects) as a trait (CleanedDocument).
• Implement the cleaner feature flag to enable the microformats::extensions::CleanedDocument trait.
• Add experimental implementation of parsing the <picture> tag.
• Allow other fields in the relation parser to be a list of values.

let obj = microformats::from_html("...")?;
let cleaned_obj = obj.as_cleaned();