Allow user to set config_file via environment variable

[botan]

Currently, the default value for the config_file argument is "~.slackr"

slackr_setup(
  channel = "#general",
  username = "slackr",
  icon_emoji = "",
  incoming_webhook_url = "",
  token = "",
  config_file = "~/.slackr",
  echo = FALSE,
  cache_dir = ""
) 

and if a user need to use another path, they have to set it in every function call explicitly e.g.

slackr_setup(config_file = "/etc/slackr")

however, this could be easily solved by setting an environment variable in .Renviron and passing it while keeping the default value the same like below

slackr_setup(
  channel = "#general",
  username = "slackr",
  icon_emoji = "",
  incoming_webhook_url = "",
  token = "",
  config_file = Sys.getenv("SLACKR_CONFIG_FILE", unset = "~/.slackr"),
  echo = FALSE,
  cache_dir = ""
) 

Happy to make a PR if you think that's okay.

[mrkaye97]

Great idea! If you're willing to submit a PR I'd be happy to review + get it shipped. Thanks!

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[mrkaye97]

Still need to merge in -- will get around to this in a bit