First draft of template verification RFC

I don't think this one would work:

```ruby
file "/tmp/foo" do
  verify "nginx -t -c #{path}"
end
```

The reason is that we don't want to run `nginx -t -c /tmp/foo`, we want to run this against the temporary file chef creates *before* we move it to `/tmp/foo` (e.g., on OS X it'd be something like `nginx -t -c /var/folders/c5/_9tvqbxd5730d8pjq9cmnnyh0000gq/T/a-thing-chef-made20140929-88016-1lfzun8`).

What Noah is suggesting is that we use a format string to generate the final value. From the ruby docs on `String#%`:

```ruby
"foo = %{foo}" % { :foo => 'bar' } #=> "foo = bar"
```

So if we pick a name for the format string placeholder, then our code is like:

```ruby 
"nginx -t -c %{filename}" % {:filename => tempfile_path} #=> "nginx -t -c /var/folders/c5/random_stuff
```

Personally, I can't decide between the two options presented; I like the similarity with xargs and find that you have with just `{}`, but I like the explicitness of `%{filename}`, too.