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/processing/base.py	Sun May 03 23:45:32 2015 -0700
+++ b/piecrust/processing/base.py	Sun May 03 23:59:46 2015 -0700
@@ -132,7 +132,7 @@
         self.out_dir = out_dir
         self.force = force
 
-        tmp_dir = app.cache_dir
+        tmp_dir = app.sub_cache_dir
         if not tmp_dir:
             import tempfile
             tmp_dir = os.path.join(tempfile.gettempdir(), 'piecrust')