Implemented configuration frame work

Added reading and writing machinery's configuration file.
The file machinery_config.rb contains the default values
used when no configuration file is present.

What about making the type explicit by adding a `type` attribute instead of inspecting the default value?

The definition of entries would then look like that:
```ruby
entry("hints",
  type:        :boolean,
  default:     true,
  description: "Show hints about usage of Machinery in the context of the commands ran by the user"
)
```