Added documentation of buildout:directory option.

Added support for extending configurations through multiple
configuration files.

Added command-line options to:

- Specify a configuration file

- Override configuration options

Renamed several options to use -s rather than _s.


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@68604 62d5b8a3-27da-0310-9561-8e5933582275