Add Agent#short_type.
Fix a leftover of JsonPath to Liquid conversion.
EventFormattingAgent: Enable access to upstream agent info via "agent".

With this enhancement you can, for example, embed the site URL in an
event created by WebsiteAgent by interpolating like
"{{agent.options.url}}", or add a message prefix depending on the type
of upstream agent using the "case" construct of Liquid.
Remove the "skip_agent" option from EventFormattingAgent.

It is now possible to include an agent name without the special flag
just by putting `"agent" => "{{agent.type}}"` in "instructions".

This commit includes automatic migration.
Fix specs after the transition from skip_agent to {{agent}}.
Add specs for AgentDrop.
Fix grammar.
Make {{agent}} universally accessible from any agent.

To implement this, EventDrop is introduced to represent an Event object
in interpolation.
Add a hover-help for the Options field which explains interpolation.
EventFormattingAgent: Partial revert: Do not include "agent" to payload.
Introduce a new concern LiquidDroppable for refactoring.
Merge pull request #410 from knu/provide_access_to_agent

EventFormattingAgent: Provide access to upstream agent
WebsiteAgent: Introduce per-node XPath evaluation in extraction.

The new extraction sub-hash key `value` holds an XPath expression which
should be applied to each node to get the result value.

The previous directives `"text": true` and `"attr": "src"` are now
written as `"value": "text()"` and `"value": "@src"`, respectively.

With this enhancement, it is now possible for this agent to perform some
basic text processing on its own by making use of XPath functions like
`normalize-space`, `substring-after` and `translate`.
WebsiteAgent: Add a spec for XPath returning an integer value.
WebsiteAgent: Improve readability with regard to XPath evaluation.
`"text": true` should have meant ".//text()", not "text()".
Correct an XPath in description.
Look into options['extract'] for checking the extraction type.

WebsiteAgent#extraction_type was private and uncallable, and may not
remain existent in future.
Added 'suggest an agent' thread link to README.md
Merge pull request #412 from knu/website_agent-per_node_xpath

WebsiteAgent: Introduce per-node XPath evaluation in extraction.