Add plugin template

[nezuo]

Adds a new plugin template to the init command: rojo init --kind plugin.

The model template can be used for a plugin but it has a few disadvantages:

• The default script is a ModuleScript instead of a Script
• The default script's code is irrelevant to a plugin:
  

  rojo/assets/default-model-project/src-init.lua

  Lines 1 to 5 in fe87472

  	return {
  	hello = function()
  	print("Hello world, from {project_name}!")
  	end,
  	}

• It does not show the new plugin build flag in the README (Add plugin build flag #735)
• The .gitignore only ignores place lock files but it should ignore the plugin model file

In the README, I suggest using .rbxm instead of .rbxmx unlike the old templates. Does anyone disagree with this change? Should we update the old templates to suggest this as well?

[boatbomber]

Yeah, .rbxm is stable now and the more efficient option. I haven't used .rbxmx in a while and we probably shouldn't still be recommending it as the default option.