caching: Use separate caches for config variants and other contexts. * The `_cache` directory is now organized in multiple "sub-caches" for different contexts. * A new context is created when config variants or overrides are applied. * `serve` context uses a different context that the other commends, to prevent the `bake` command's output from messing up the preview server (e.g. with how asset URLs are generated differently between the two). * Fix a few places where the cache directory was referenced directly.

--- a/piecrust/commands/base.py	Sun May 03 23:45:32 2015 -0700
+++ b/piecrust/commands/base.py	Sun May 03 23:59:46 2015 -0700
@@ -19,6 +19,7 @@
         self.name = '__unknown__'
         self.description = '__unknown__'
         self.requires_website = True
+        self.cache_name = 'default'
 
     def setupParser(self, parser, app):
         raise NotImplementedError()