facets-preview shiny app execution on linux

[anoronh4]

we were hoping to containerize this facets preview in our docker that has the base rocker/tidyverse (has a number of R packages pre-installed), however we are having difficulties based on the fact that shiny app was developed specifically for mac. for example:

facets-preview/inst/application/server.R

Line 73 in 0d89717

last_mod = as.numeric(system(paste0("stat -f%c ", values$config$watcher_dir, "/watcher.log"), intern=TRUE))

stat -f%c <file> works only on mac. i would suggest that there be linux alternatives so that the linux equivalent stat -c%Y <file> executes when the OS is linux. Sys.info()['sysname'] can be executed to determine the os name, alternatively OS can be indicated as a parameter in the config file.

i'd be happy to make the PR myself pending your thoughts.