Added a `plugin install-all` command that reads from a plugins.json file...

... previously exported from `plugins list` and attempts to install from Wordpress plugins repository.

Currently it will only install plugins marked as active and does not consider any plugins already installed. Its primary use is to bootstrap a new instance based on an existing plugins.json

It will also try to install the specific plugin version listed in plugins.json, but  will default to the latest version if the specific version cannot be found.