Remove run_heroku and replace with logic to inspect env vars for server deployments

[rpkyle]

Currently, DashR provides two separate methods for launching the fiery server and serving up a Dash app. The first is run_server():

dashR/R/dash.R

Lines 414 to 418 in 3ff3c62

	run_server = function(host = NULL, port = NULL, block = TRUE, showcase = FALSE, ...) {
	if (!is.null(host)) self$server$host <- host
	if (!is.null(port)) self$server$port <- as.numeric(port)
	self$server$ignite(block = block, showcase = showcase, ...)
	},

...and the second is run_heroku():

dashR/R/dash.R

Lines 419 to 423 in 3ff3c62

	run_heroku = function(host = "0.0.0.0", port = Sys.getenv('PORT', 8080), ...) {
	self$server$host <- host
	self$server$port <- as.numeric(port)
	self$run_server(...)
	}

These differ predominantly in the way they set and handle default port and host variables. It would be preferable to condense these into a single method called run_server(), and attempt to autodetect environment set sensible defaults if the app is deployed to DDS or Heroku and no defaults values are supplied for these arguments.

[rpkyle]

Following discussion with @chriddyp, we should examine the local environment for variables DASH_PORT and DASH_HOST if host and port are not explicitly passed in by the app developer.

If neither exists, and no values are provided for these parameters, we should use 127.0.0.1 for host and 8050 for port, for parity with Dash for Python.

[rpkyle]

run_heroku was 🔪 in #75. Closed.