"include_dir" attribute on Theme for overrides by configuration

[preaction]

Instead of bundling a theme if we just need to override some includes, allow an include_dir key that is a single string or an array of strings which are interpreted as paths to include.

[preaction]

Being able to specify just an extra include directory would solve this better.

[preaction]

I really prefer just having the user bundle their theme. No muss, no fuss, no bother. Then they aren't subject to the whim of upgrades.

I only have this problem because I'm the one developing my own theme, which are mostly clones of the default theme.

[preaction]

Having an include_dir attribute is best, making a bunch of things possible with just some CSS and HTML snippets.

[preaction]

But they aren't dirs, they're stores. And maybe I need to stop this charade of using HDFS or GridFS or WebDAV for Stores...

[preaction]

If I use the include_dir attribute (#210), can I add other, additional CSS files perhaps?