Write a custom rake task to generate a `dot` or Mermaid ERD file

[postmodern]

Related to issue #18, we could write a custom rake task that loads the Ronin::DB models and outputs a dot or Mermaid ERD file, which can then be used to generate output SVG or HTML. See this example code.

This code will likely get very complex, and should probably be stored in a separate file in tasks/ then loaded into Rakefile using require_relative.