Add odt_document2 and powerpoint_presentation2

[atusy]

This PR adds odt_document2 and powerpoint_presentation2 and closes #722 #741
Though I'm not sure powerpoint_presentation2 is a scope of bookdown, bookdown is the only place to implement it.

odt

I rendered inst/examples/index.Rmd.
The result seems to be fine.

bookdown.zip

Exceptionally, when math expressions appeear right after the code blocks, the math expressions appear within code blocks.
However, this is not a problem of bookdown because the same problem occurs in rmarkdown::odt_document.

pptx

Cross reference seems to be working fine

---
output: bookdown::powerpoint_presentation2
---

# Cross-reference

```{r iris, fig.cap="iris"}
plot(iris)
```

`\@ref(fig:iris)` becomes \@ref(fig:iris)

[atusy]

May be it's better to factor out duplicated codes into a new function (office_document2_base?) from word_document2, powerpoint_presentation2, and odt_document2?

[yihui]

Yes, that is exactly what I was thinking. Please do factor out the code instead of repeating it in three functions. Thank you!

[atusy]

Done it by implementing office_document2_base.
It is not exported currently, but might be useful for users in case they want some formats to be transformed to bookdown's format.

As rewriting extremely simplfied odt.R and powerpoint.R, I integrated them into word.R.
Maybe we should renaming word.R to office.R or somethingl like that.

[atusy]

I confirmed markdown_document2 well works with other formats (e.g., prettydoc::html_pretty.
Note that hyperlinks to referred tables and others are not working.

---
output: 
  bookdown::markdown_document2:
    base_format: prettydoc::html_pretty
---

# Table \@ref(tab:iris)

```{r iris}
knitr::kable(head(iris), caption = "iris")
```

# Table \@ref(tab:mtcars)

```{r mtcars}
knitr::kable(head(mtcars), caption = "mtcars")
```

[yihui]

Perfect. Thank you so much!

[jooyoungseo]

I confirmed markdown_document2 well works with other formats (e.g., prettydoc::html_pretty.
Note that hyperlinks to referred tables and others are not working.

---
output: 
  bookdown::markdown_document2:
    base_format: prettydoc::html_pretty
---

# Table \@ref(tab:iris)

```{r iris}
knitr::kable(head(iris), caption = "iris")

Table @ref(tab:mtcars)

knitr::kable(head(mtcars), caption = "mtcars")

@atusy, bookdown::markdown_document2 does not seem to work with redoc package.

Cross-references are broken in the rendered results. Would you mind testing it with the following?:

---
output: 
  bookdown::markdown_document2:
    base_format: redoc::redoc
---

Table \@ref(tab:iris)

```{r iris}
knitr::kable(head(iris), caption = "iris")
```

Table \@ref(tab:mtcars)

```{r mtcars}
knitr::kable(head(mtcars), caption = "mtcars")
```

Figure \@ref(fig:histogram)

```{r histogram}
hist(airquality$Ozone, col="blue")
```

[atusy]

As far as I tried, broken cross-references occur only at Table \@ref(tab:iris).
If keep_md: true is specified, you see the line is converted to

Table \<span class="redoc" id="redoc-citation-1"><span class="redoc" id="redoc-citation-2"><span class="redoc" id="redoc-citation-3">@ref</span></span></span>(tab:iris)

I think this is the reason and is caused by redoc package.
Maybe @noamross knows what is happening.

Besides, Figure \@ref(fig:histogram) is converted to Figure ?? in docx.
This is because you did not specify fig.cap for the histogram chunk.

BTW, I recommend opening issue when you see strange behavior instead of making comments on closed PR.
Then, more people notices the issue and possibly solve it more quickly.
(This is just my opinion)

[noamross]

This is most likely caused by the redoc package as I haven't yet done the work to make sure its post-processing is compatible with these formats. Feel free to open an issue over there.