consider geojsonsf

I see you're using `geojsonio` to convert `sf` objects to GeoJSON. Would you consider using [`geojonsf`](https://github.com/SymbolixAU/geojsonsf) to improve speed?

Hopefully in this benchmark I'm replicating what you're doing inside [topogram](https://github.com/dreamRs/topogram/blob/6b3d1b66fbaadc273eaacc60780c4335daf194ae/R/topogRam.R#L159)

```r
library(microbenchmark)
library(geojsonsf)
library(geojsonio)

microbenchmark(

geojsonsf = {
shape$id <- 1:nrow(shape)
sf_geojson( shape )
},
geojsonio = {
geo_list <- geojson_list(input = shape)
for (i in seq_along(geo_list$features)) {
geo_list$features[[i]]$id <- i - 1
geo_list$features[[i]]$properties$id <- i - 1
}
geo_json <- geojson_json(input = geo_list)
},
times = 5

)
# Unit: milliseconds
# expr min lq mean median uq max neval
# geojsonsf 1.955478 2.078431 2.172361 2.126749 2.226844 2.474302 5
# geojsonio 235.419525 239.354627 241.348199 240.610821 242.299392 249.056631 5
```

Note, I'm planning an update to `geojsonsf` by the end of October with some important fixes & updates.

---

You can also specify package version dependencies inside the `DESCRIPTION` in place of inside functions

```r
if (packageVersion("geojsonio") < "0.6.0.9100")
stop("You need geojsonio >= 0.6.0.9100 to use this function.", call. = FALSE)
```

vs

```
Imports:
htmlwidgets,
geojsonio (>=0.6.09100),
htmltools,
shiny,
rmarkdown,
jsonlite,
magrittr
```

This way you don't have to implement the version check inside each function which may require it.