Work inside google colab

Hi,

Would it be possible to make the package work correctly inside google colab? The problem I noticed (there may be others) is that `.turtle_undraw()` uses `grid.remove()`, which causes colab to create a new graphic area at every call. I tried to change `.turtle_undraw()`:

```
.turtle_undraw <- function()
{
grid.remove("turtle_head", redraw=FALSE)
grid.remove("turtle_body", redraw=FALSE)
grid.remove("turtle_leg1", redraw=FALSE)
grid.remove("turtle_leg2", redraw=FALSE)
grid.remove("turtle_leg3", redraw=FALSE)
grid.remove("turtle_leg4", redraw=FALSE)
invisible(NULL)
}
```

But this does not remove/undraw the turtle. Eliminating `redraw=FALSE` from just the last line does remove/undraw the turtle and reduces the number of bogus graphic areas, but that is not a solution.

As a workaround, I eliminated the turtle image completely:

```
library("TurtleGraphics")
fakedraw <- function() {invisible(NULL)}
environment(fakedraw) <- asNamespace('TurtleGraphics')
assignInNamespace(".turtle_draw",fakedraw, ns="TurtleGraphics")
assignInNamespace(".turtle_undraw",fakedraw, ns="TurtleGraphics")
turtle_init()
```

which is obviously not great but apparently works ok, at least for simple stuff.

Thanks!