require pry-byebug in cli.rb which Pry will load in Pry.init

I ran into an issue where I would get the error "ArgumentError: uncaught throw :breakout_nav" if I do this:

```ruby
require 'pry'
# require 'pry-byebug'
binding.pry
puts "next statement"
[1] pry(main)> next
```
This pull request resolves that by using cli.rb to load pry-byebug before pry-core has a chance to call the un-extended Pry.start.  I can follow up with more details if needed.