Prior to this commit, if a user ran the `vagrant ssh -c CMD` command, it would not allow the user to configure pseudo-terminal allocation. This commit introduces a -t flag for the `vagrant ssh` command which defaults to true if not specified.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.