Added beamer_presentation2

[jooyoungseo]

This PR resolves #790 by consulting with this StackOverflow question and answer.

Change

• Added bookdown::beamer_presentation2() output format.

Testing

I have confirmed the following minimal template working as expected.

---
output: bookdown::beamer_presentation2
---

# Table Test

See Table\ \@ref(tab:test-table).

```{r test-table}
knitr::kable(head(airquality), caption = "Test table.")
```

---

# Figure Test

See Figure \@ref(fig:test-figure).

```{r test-figure, fig.cap="Figure test."}
hist(airquality$Ozone, col="blue")
```

[yihui]

Thank you!