log_msg: Helper function to print out log messages into Shiny
using cat() and stderr(), as described on
https://shiny.rstudio.com/articles/debugging.html
Description
Because this can print a lot, it's silent unless
the shiny.router.debug option is set.
Usage
log_msg(...)
Arguments
...
All params get passed through to cat().
They're automatically wrapped in shiny::isolate(),
so you can print reactive values here without too
much worry.