Fix win-devel ERROR

[k5cents]

Per the CRAN checks, the package produces an ERROR on r-devel-windows-ix86+x86_64:

* installing *source* package 'gluedown' ...
** package 'gluedown' successfully unpacked and MD5 sums checked
** using staged installation
** R
** inst
** byte-compile and prepare package for lazy loading
Error in loadNamespace(i, c(lib.loc, .libPaths()), versionCheck = vI[[i]]) : 
  there is no package called 'stringi'
Calls: <Anonymous>...loadNamespace -> withRestarts -> withOneRestart -> doWithOneRestart
Execution halted
ERROR: lazy loading failed for package 'gluedown'
* removing 'd:/Rcompile/CRANpkg/lib/4.0/gluedown'
* restoring previous 'd:/Rcompile/CRANpkg/lib/4.0/gluedown'

This issue stems from the inability to install the {stringi} package used by {stringr}. Might need to look into conditional usage? Looks to be related gagolews/stringi#320.