Support fig.show='hold' for office output

[atusy]

When output is office formats (e.g., docx) and fig.show='hold', then knitr coerces fig.show='asis' with warning.
This is introduced by f6e3b62#diff-4910cdfe600c3874a655c6951e12daa0 to fix rstudio/bookdown#249 and is expanded by 575e18e#diff-4910cdfe600c3874a655c6951e12daa0 .

However, regardless of the coercion, fig.show='hold' seems working.
In addition, removing the coercion still works.
I do not face error reported by rstudio/bookdown#249

Although I don't know what made things possible, can we remove the relevant lines?
If not, I think we should also add the treatment here: https://github.com/yihui/knitr/blob/master/R/hooks-md.R#L20

Example Rmd

---
title: "Untitled"
output :
  word_document: default
  powepoint_presentation: default
  odt_document: default
  rtf_document: default
  bookdown::word_document2: default
---

# fig.show='hold'

```{r base, fig.show='hold', fig.cap=.cap}
.cap = c('a', 'b')
plot(1)
'a'
plot(2)
```

# fig.show='asis'

```{r ref.label='base', fig.show='asis', fig.cap=.cap}

```

BTW, when fig.show='hold', captions are not working in the above example.
I'm currently working to fix this.

xfun::session_info('knitr')

R version 3.6.1 (2019-07-05)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux 9 (stretch), RStudio 1.2.1335

Locale:
  LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8   
  LC_MESSAGES=C              LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
  LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

Package version:
  evaluate_0.14   glue_1.3.1      graphics_3.6.1  grDevices_3.6.1 highr_0.8       knitr_1.24.4    magrittr_1.5    markdown_1.1    methods_3.6.1  
  mime_0.7        stats_3.6.1     stringi_1.4.3   stringr_1.4.0   tools_3.6.1     utils_3.6.1     xfun_0.9        yaml_2.2.0     

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[atusy]

Although I guess the change is caused by knitr or other R packages, I tried Pandoc 2.2.1 and 2.7.3 just for sure.
Both works fine.
I gonna make PR.

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