Documentation helper for define_hook from ruby-hooks

Adds support for automatic detection of define_hook, sets @return type to Hook, allows @notify_param documentation tag to describe parameters called by Hook#notify_objects.

ruby-hooks comes with a support gem that allows to write yard docs for the hooks.

To automatically use the plugin add --plugin ruby-hooks to projects root directory .yardopts file.

Example documentation:

# called before executing command
# @notify_param object [Object] the target that invoked the method
# @notify_param stdout [String] standard output of the command, can be nil
# @notify_param stderr [String] standard error output of the command, can be nil
define_hook :on_execute_data

All hooks will be listed in Hooks section of the documentation, and the generated hook description will look roughly like this:

` - (Hook) on_execute_data`

called before executing command

**Returns:**

- (`Hook`) — the Observable instance

Hook#notify_objects block params:

- **object** (`Object`) — the target that invoked the method
- **stdout** (`String`) — standard output of the command, can be nil
- **stderr** (`String`) — standard error output of the command, can be nil