Added configuration option website.base_url

The base URL is needed when relative URLs won't work and
extensions need to create absolute URLs.
Updated Webgen::TestHelper#setup_default_nodes to allow specifying the Node class
Updated path handlers feed and sitemap to use new website.base_url config option
Extracted BundleLoader#load! from #load to allow force-loading of files

This facility is now used to load the configuration file without
registering it as a bundle. It can also be used to load additional
initialization code in bundles.
Fixed overwriting of context[:config] when processing a tag
Added warning if a specified tag template node could not be found
The default node write order (sort by alcn) can now be modified by using the write_order meta information
Added YAML::SyntaxError to list of caught exceptions when parsing YAML
Fixed tests so that they work even if an optional library is not installed
Moved all CLI commands into lib/webgen/cli/commands and fixed all non-working ones
Fixed typo in config option example
Added information about config option documentation to generated bundle info.yaml file
The CLI command show config now optionally displays additional information (syntax, examples)
Added content processor rainpress
Enable ANSI colors in CLI on Windows if ansicon is running

Win32/Console/Ansi was deprecated in favour of ansicon.
See https://github.com/adoxa/ansicon
Allow forced creation of a new Website object in ClI::CommandParser#website
Added a simple implementation for auto-generating the website
Fixed implementation/tests so that the test suite works on Windows
Removed copied commented, useless lines
Fixed source file_system to work on Ruby 2.0

Due to a change in how Dir.glob does its thing, webgen was broken.
I have fixed it and also filed a bug report.