Feat/add markdown input

[CarloGauss33]

Changes

This PR adds markdown rendering through Redcarpet gem.

In particular, it adds MarkdownBuilder that exposes the markdown_column and markdown_row renderers in active admin.

Example:

Using the following input

and using the builder in the admin

show do
  attributes_table do
    markdown_row(:description)
  end
end

Results in: