feat(vim): improve markdown editing with long wrapping lines

In markdown I have long lines wrapping as we are often editing docs that
don't stick to 80 columns. This makes it hard to navigate with j and k
imposable to use becase is skips over the wrapped lines.

This now using gj and gk to navigate wrapped lines so j and k will act
naturaly and move down one "line" even on the wrapping.

site-modules/core/files/vim/after/ftplugin/markdown.vim

@@ -1,2 +1,8 @@
 set wrap
 set conceallevel=0
+
+
+nnoremap j gj
+nnoremap k gk
+
+set linebreak