Fix parsing the `@` prefix for botname mention

```
>> def strip_name(text, nick=nil) text.gsub(/\A(#{nick}:|@#{nick}) /, '') end
```
Test cases:
```
>> strip_name("pagerbot: make me a sandwich", "pagerbot")
=> "make me a sandwich"
>> strip_name("@pagerbot make me a sandwich", "pagerbot")
=> "make me a sandwich"
>> strip_name("pagerbot make me a sandwich", "pagerbot")
=> "pagerbot make me a sandwich"
```