Add a Ronin::Recon::Config#to_yaml method

[AI-Mozi]

Closing #159
I don't know how (and if it's even possible) to save workers with indents :/

[postmodern]

Overall looks good, just minor suggestions.

I may have realized a problem with my Config::Workers#initialize implementation. Since it converts the input value to a Array, and we dump that Array value back out to the YAML file, this means the workers: will always end up being an explicit Array of worker IDs. This means that when we release a new version of ronin-recon, which adds new default built-in workers, users won't have those workers enabled by default because their config file will contain an explicit Array of worker IDs. I think I need to refactor Workers#initialize to preserve the original value so we load a Hash of worker_id: true|false and save the workers: back to a Hash. I'll need to think about this.

[postmodern]

Noticed one last thing. Looks good though.

I will need to re-think how to properly store the list of workers as a Hash by default.

[postmodern]

Missing @since tags on new methods.