Added underscore plugin

This plugin removes all symbols that begin with an underscore from doc output. This comes in handy when you're using underscores to indicate private variables that you want documented in code or internal docs, but not in published api documentation.
Nit: For consistency with other files, please use Stroustrup braces:

```js
if (foo) {
    return true;
}
else {
    return false;
}
```