Added more load_cache() arguments to avoid clobbering global environment

[LTLA]

Bioconductor's OSCA book uses load_cache() to pull knitr-cached content from already-compiled chapters to enable their re-use when compiling later chapters. By and large, this works pretty well but load_cache() has the unfortunate side effect of loading everything in the labelled chunk into the global environment, not just the object I asked for. To illustrate, let's say we have a very simple Rmarkdown document:

---
title: Testing
---


```{r blah, cache=TRUE}
var1 <- 1
var2 <- 2
```

When I compile it with rmarkdown::render, restart the R session and load the chunk, I get:

knitr::opts_knit$set(output.dir=".")
knitr::load_cache(path="blah_cache/html/", label="blah", object="var1") # good
##  [1] 1
var2 # I didn't ask for this
## [1] 2

This can be annoying when an object in my current environment (in an interactive R session, or during the course of compiling an Rmarkdon document) is silently and unintentionally overwritten. The solution presented in this PR adds a few arguments to load_cache() that give the caller a bit more control. The first is an envir= option that defines a workspace for cache loading, other than the default environment from knit_global().

knitr::load_cache(path="blah_cache/html/", label="blah", object="var1", envir=new.env())
## [1] 1
var2
## Error: object 'var2' not found

The second argument (dir=) is just a quality-of-life change so that load_cache() can be more easily called outside of a knitr context (e.g., in an interactive session), specifically by ignoring the (lack of) an output.dir setting.

In both cases, the defaults should preserve the current behavior of the function.

Session information

R version 4.0.0 Patched (2020-05-01 r78341)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.5 LTS

Matrix products: default
BLAS:   /home/luna/Software/R/R-4-0-branch-dev/lib/libRblas.so
LAPACK: /home/luna/Software/R/R-4-0-branch-dev/lib/libRlapack.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] knitr_1.30.1

loaded via a namespace (and not attached):
[1] compiler_4.0.0 tools_4.0.0    xfun_0.18     

[CLAassistant]

All committers have signed the CLA.

[yihui]

I definitely agree this is an annoying problem and should be fixed. I'll take a closer look at your PR tomorrow. Thanks a lot!

[LTLA]

Whoops, lost track of this. Any thoughts?

[yihui]

Sorry for dropping the ball. I'll try my best to include this PR into the next version of knitr!

[yihui]

The changes look good to me. I'll do a few cosmetic changes and merge later. Thank you!