Add option for not copying default td-agent.conf

I am using chef-td-agent but I don't want to use the default
matchers and sources defined in
https://github.com/treasure-data/chef-td-agent/blob/master/templates/default/td-agent.conf.erb

In order to remove them I do something like

```
include_recipe "td-agent"

template "/etc/td-agent/td-agent.conf" do
  mode "0644"
  source "td-agent-without-defaults.conf.erb"
end
```

This is kind of undesirable because everytime I provision a machine
with this recipe, the resource is updated by chef-td-agent and then
by my recipe, which causes td-agent to restart twice (even when there
were no differences in the file in consecutive provisionings).

I would be happy if this could be included in your repo.