A plugin system via sub-generators

[drublic]

This is the bug to continue the discussion from use-init/init#67.

I wrote up an outline on how we can achieve the goal:

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I think a decent way would be to clone the plugin to install in a directory, move all the things around and delete the rest of the repo. This would mean that we only need a convention for

1. repo URLs
2. file names
3. directories

Most of this is given:

1. Repos for files can be matched as "use-init/init-" at GitHub. We could later extend this to make it customizable and stuff.

__ 2.__ I've outlined this above: We need to copy the {taskname}.js file into the folder grunt/plugins, we need to run npm install --save-dev {pluginname} and lastly add necessary config options to main project's config.js which are stored in a plugin specific config.js file.

3. We need a temporary folder to place the repos in. I'd suggest to name it .init-cache. Everything else is stated in 2..

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cc: @distilledhype @sergiovilar

[khalillechelt]

Hey @drublic thanks for this. I don't have time to do this right now...

@sergiovilar do you have time for this?

[khalillechelt]

This should be taken care of before: #19

[khalillechelt]

And this as well: #21