Simplify plugin configuration

Forcing the configuration into a hash doesn't work very well and doesn't
yield any noticeable benefits.

This change lets role users specify plugin configuration in the same
format used by collectd configuration files, as a text block.  This way
any configuration structure can be used without the need for
workarounds.

Also clean up the examples in the README to be more idiomatic YAML.
Merge pull request #5 from waysact/simple-config

Simplify plugin configuration