Key-value lookup

[neelsmith]

• [x] Look up by key alone should return a vector of page-value pairs across the vault. Signature: single string parameter, key.
• [x] For an optional page-list parameter, the retrieval would be limited to that set up of pages. Signature: key, Vector of page names, vault
• [x] For key + value, should return pages with key/value pair. Signature: key, value, vault