Add logic to add Git based components

[bobbingwide]

Having been successful updating Git based components it makes sense to also be able to add them.

Requirements

• Install a plugin or theme from a public Git repository
• Require the following information: URL to Git repository, component type
• Flag to indicate whether or not the component should continue as a git repository.

Proposed solution

• Add a dialog to the oik_batch admin page
• Check that the component does not already exist.
• Use git clone to create the component.

Notes

• Activation, deactivation, deletion and settings to be managed using normal admin interface.
• Consider supporting MU plugins.
• Not suitable for plugins that need to be built before they can be used.