Doc parsing fails on @option notation

Adding the following 

```
# @option test_hash [String] :test_opt a Hash test argument
```

to `test/rubycom/util_test_module.rb`
and running `./test/rubycom/util_test_bin.rb UtilTestModule test_command_arg_hash --help`
results in the following exception

```
helpers.rb:17:in `block in format_tags': tag[:types]  should be an Array but was NilClass (ArgumentError)
```
