Loving this gem!

I wish I could pull the icons in the toolbar from my own iconset so they match the rest of my site's look and feel. I tried overriding Marksmith::Helper#marksmith_toolbar_button but it looks like the one in the engine is used before any helper defined in the Rails app.

I'm not sure how to go about this in an elegant way...maybe a config hash that takes the name of the function (bold, italic) and maps it to a proc, which you can define to pull an image or an SVG or anything else? Now sure how well this plays with the asset pipeline...maybe it could be configured to use a named Rails helper first, if defined, and if not fallback to the existing image_tag and included SVG icons?

I did discover that if I copy views/marksmith/shared/_editor.html.erb to my own codebase then I can change anything I want, including the icons! However, this makes it much more difficult to stay in sync with updates from the main gem. ☹️