add stats_options for YardocTask, simplifies rake task definitions for extra stats options

this simplifies:
```ruby
YARD::Rake::YardocTask.new do |t|
  t.files   = ["lib/**/*.rb"]
  t.options = ["--no-stats"]
  t.after = Proc.new do
    YARD::CLI::Stats.new.run("--list-undoc", "--compact")
  end
end
```
to:
```ruby
YARD::Rake::YardocTask.new do |t|
  t.files   = ["lib/**/*.rb"]
  t.stats_options = ["--list-undoc", "--compact"]
end
```