formd is a tool for (for)matting (M)ark(d)own that allows rapid conversion between the two styles of Markdown links and images -- inline and referenced.
This is pretty great little utility. I'm using it from vim to convert inline links to reference links. It runs every time I save a Markdown file thanks to this line in my .vimrc:
autocmd BufWrite *.md :%! ~/bin/formd -r