Alter each engine to spawn its head and foot pattern into _meta/ if not found

[bmuenzenmeyer]

part of #603

• mustache
• handlebars
• twig
• underscore
• nunjucks Add support to spawn _meta header and footer patterns if not found patternengine-node-nunjucks#2
• react https://github.com/pattern-lab/patternengine-node-react/issues

[bmuenzenmeyer]

would be great if anyone else could help me understand what Underscore or React expect for these patterns

[bmuenzenmeyer]

paging @geoffp - we discussed this what feels like ages ago, but can you ping me where I can find the underscore meta patterns?

[geoffp]

@bmuenzenmeyer actually, I don't think we ever made any. It looks like in our underscore project, we're still using the default-ish mustache meta patterns, even though underscore is the main engine in use.