Environment-specific app config files

[alanpich]

Allow loading environment-specific config files from the ./config directory.

./config/slender.yml   - global config
./config/slender.PRODUCTION.yml
./config/slender.DEVELOPMENT.yml

[silentworks]

These should override properties in the slender.yml e.g. If I have my routes defined in slender.yml, I wouldn't want to have to define them again in slender.PRODUCTION.yml. This should only override the properties set in slender.yml with the new properties set in slender.PRODUCTION.yml.

[alanpich]

definitely also need a 'slender.local' config file to allow overrides that won't be committed into source control (for my local dev db connection etc, or sendmail overrides etc)