@emeralt/cli caching proxy

[stackdumper]

Currently, emeralt-cli runs emeralt-server with database-inmemory and storage-inmemory, redirecting all non-existent package requests to registry.npmjs.org. This behavior has a little use for developers.

Instead, emeralt-cli should provide caching proxy server which would redirect and cache all read requests to registry.npmjs.org. Caching should be provided by a plugin, thus server should provide a plugins system.