Merged the migrations, import and export CLIs into the main Contentful CLI

The functionalities provided by contentful-import, contentful-export, and migration-cli are now all available through the contentful-cli. With only one tool to install, discovering Contentful features from the command line is much easier now. The Contentful CLI can authenticate with Contentful and pick a default space so you’ll no longer have to keep passing around space identifiers and access tokens for much of your work.

The three former CLI’s will remain usable as Node libraries but they will no longer be installable for global usage as CLIs.