Copying and pasting visual selection to REPL

Currently `:ToggleTermSendVisualSelection` sends lines to the REPL line-by-line, which in many cases breaks the behavior of code blocks. For example, when sending the following code to ipython
```
def hi():
print("hi")

return "message sent"
```
we get a `SyntaxError: 'return' outside function` because, being sent line-by-line, the line break above starts a new execution cell (actually, you first get an indentation error because the `print` command is sent by default without indentation; #243 provides a quick fix). These errors would be fixed by literally copying and pasting the above code into the REPL, rather than sending it line-by-line. Could you consider implementing this behavior (or suggesting how to do so)?