Allow merging edges in mermaid output

Closes #27 (closed)

Example

require Grains
recipe = Grains.new_recipe(Test, %{
  Producer => [
    Grains.route(:some_message, [Consumer]),
    Grains.route(:another_message, Grains.periodic(Consumer, 1000))
  ]
})

Merging disabled

iex(5)> Grains.Recipe.to_mermaid(recipe) |> IO.puts()

graph TD
Producer -->|":some_message"| Consumer
Producer -->|":another_message, periodic"| Consumer

Merging enabled

iex(6)> Grains.Recipe.to_mermaid(recipe, merge_edges: true) |> IO.puts()

graph TD
Producer -->|":another_message, periodic<br />:some_message"| Consumer