Implemented `Router#define`

:smiling_imp: 
Do you think I'm fine with those **identical** lines of code? :trollface: 

```ruby
def initialize(options = {}, &blk)
  # ...
  instance_eval(&blk) if block_given?
end

def define(&blk)
  instance_eval(&blk) if block_given?
end
```

Let's do

```ruby
def initialize(options = {}, &blk)
  # ...
  define(&blk)
end
```

:wink: