added configurarations options

[francescob]

Added a configuration key called models_list, in case someone needs to draw a chart for a subset of the models in the rails application.

example configuration:

models_list: User.descendants

default is still ::ActiveRecord::Base.descendants.sort_by(&:name)

the method could be anything that will return an array of active record models or even a string containing an array of models:

"[User, Role]"

Also added

hide_columns: true

to hide columns by default for readability purpose

not settings the option or setting it to false will show columns like it is in the current setting