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.

colorama==0.3.3
compressinja==0.0.2
Jinja2==2.7.3
Markdown==2.6.2
MarkupSafe==0.23
Pygments==2.0.2
pystache==0.5.4
python-dateutil==2.4.2
PyYAML==3.11
repoze.lru==0.6
smartypants==1.8.6
strict-rfc3339==0.5
textile==2.2.2
Werkzeug==0.10.4