"In normal mode, [ positions the cursor in the checkbox and waits for a character with which to replace the checkmark.
au FileType todo nnoremap [ 0t]r
"In insert mode, [ actually inserts a checkbox and a space.
au FileType todo inoremap [ [<Space>]<Space>
"Any file with a name ending in todo is a todo file.
au BufRead,BufNewFile *todo set ft=todo