vim:keys: Expand visual selection on <leader>[jk]

Add two vmaps that call ExpandVisualSelection() for the appropriate
direction.

That expands the selection over all directly following lines in the
given direction that contain the current selection at the same position.

Example:

```
 - TODO: ...
 - TODO: ...
 - TODO: ...
```

In visual block one can select `TODO: ` on the first line and then call
`ExpandVisualSelection(1)` which results in a block selection that spans
over all other TODOs as well.

.config/vim/vimrc.d/40-keys.vim

@@ -287,3 +287,57 @@ nnoremap <leader><C-U> gUl
 vnoremap <leader><C-U> gU
 nnoremap <leader><C-L> gul
 vnoremap <leader><C-L> gu
+
+" Expand visual selection over all directly following lines in the given
+" direction that contain the current selection at the same position.
+"
+" Example:
+" ```
+"  - TODO: ...
+"  - TODO: ...
+"  - TODO: ...
+" ```
+"
+" In visual block one can select `TODO: ` on the first line and then call
+" `ExpandVisualSelection(1)` which results in a block selection that spans over
+" all other TODOs as well.
+function! ExpandVisualSelection(direction)
+	let l:sel = escape(GetVisualSelection(), '\')
+	normal gv
+
+	" Move the cursor onto the side of the selection that points in the
+	" direction of the expansion.
+	let l:swap_ends = 0
+	if (
+			\ (getpos('.') == getpos("'>") && a:direction < 0) ||
+			\ (getpos('.') == getpos("'<") && a:direction > 0)
+	\)
+		normal o
+		let l:swap_ends = 1
+	endif
+
+	if (a:direction < 0)
+		" Because of the greedy nature of search(), we cannot use the same
+		" regex/approach as when searching forwards, as it will only ever match
+		" the preceding line.
+		while (
+				\ (line('.') - 1) &&
+				\ match(getline(line('.') - 1), '\%'.col("'<").'c'.l:sel) != -1
+		\)
+			call cursor(line('.') - 1, col("'<"))
+		endwhile
+	elseif (a:direction > 0)
+		let l:pat = '\v%#(.*\n.*%' . col("'<") . 'c\V' . l:sel . '\)\*'
+		call search(l:pat, 'e')
+	else
+		" TODO: expand in both directions
+	endif
+
+	" Reset cursor column
+	if (l:swap_ends && mode() == "\<C-V>")
+		normal O
+	endif
+endfunction
+
+vmap <silent> <leader>j :<C-U>call ExpandVisualSelection(1)<CR>
+vmap <silent> <leader>k :<C-U>call ExpandVisualSelection(-1)<CR>