get_language_lines_in_visual_selection off by one

[benlubas]

if I have a code block like this:

```{python}
print("Hello")
x = 10
...

and call otterkeeper.get_language_lines_in_visual_selection(true) with the first line visually selected (visual or visual line mode) I get nil.

So I think there is an off by one somewhere

[jmbuhr]

That is quite likely, I have gone through several iterations where I thought I'd fixed an off by one error only to get a different one. Like putting in an usb-stick, but worse. I'll have a look!