Add carthage-layout

[srak289]

It used to feel confusing for me to setup a CarthageLayout from scratch, now it feels a bit tedious. I'm proposing this as a starting point for a utility to create new layouts and perhaps grow in the future to manage plugins.

I'm open to opinions on what a "default" CarthageLayout should present.

[hartmans]

I'd definitely like to have something like this. I think there are a few types of layouts:

• Layouts that actually can just be a class. It turns out that as things get more complicated, you often end up with a function that for example looks at config state and returns a layout
• Layouts that need to be a function

I think we don't want to hard code the templates in the carthage-new executable, but instead, have them live under carthage/resources/skel or the existing carthage/resources/templates.

I don't think we need a config.yml with a layout. A carthage_plugin.yml can include a config section, but unless you're setting plugins a generic layout doesn't even need that. (layout_name is not really required, and I wouldn't support setting base_dir and checkout_dir explicitly by default.)

[hartmans]

I wonder if ith would be better to call this carthage-new, and have it take a template like:

• layout
• layout_function
• layout_plugin (a layout that needs to have a well-defined package name so that libraries can reference it.)

[srak289]

I'm not quite sure what a default layout_function should look like. I'm happy to work against an example to implement.