Making use of environment variables and a better README

README.md

@@ -5,4 +5,41 @@
 # Reason
 
 This project is a journey of learning Golang. 
-If you would like to make an MR, please go right ahead, it would be a good learning process for me.
+If you would like to make an MR, please go right ahead, it would be a good learning process for me.
+
+# Usage
+
+`go-cmw` features a good command-line interface thanks to `kingpin.v2`.
+
+```console
+$ go-cmw --help
+usage: go-cmw [<flags>]
+
+A small terminal wrapper around the wttr.in weather endpoint.
+
+Flags:
+      --help               Show context-sensitive help (also try --help-long and --help-man).
+  -m, --metric             Display weather in metric
+  -u, --uscs               Display weather in imperial
+  -M, --meter-second       Display wind in m/s
+  -z, --zero               Show the weather now
+  -o, --one                Show the weather for one day
+  -w, --two                Show the weather for two days
+  -A, --ignore-user-agent  Request ignoring the user agent
+  -F, --follow-link        Follow link redirect
+  -n, --narrow             Display weather in narrow view
+  -q, --quiet              Add the quiet flag
+  -Q, --super-quiet        Add the super quiet flag
+  -N, --no-colors          Disable displaying colors (always enabled on windows
+  -p, --add-frame          Add a frame to the output
+  -T, --mid-transparency   Enable mid-transparency (PNG only)
+  -P, --png                Download a weather PNG image
+      --v2                 Use the v2 endpoint
+  -t, --transparency=0     Set transparency level (0-100) (PNG only)
+  -L, --location=LOCATION  Specify explicite location
+  -l, --language=LANGUAGE  Specify explicite language
+      --version            Show application version.
+```
+
+On top of the following command-line flags, `go-cmw` can also take advantage of environment variables.
+`go-cmw` can take advantage of `GO_CMW_LOCATION` and `GO_CMW_LANGUAGE` environment variables if set.

main.go

@@ -250,8 +250,8 @@ var (
 	png             = app.Flag("png", "Download a weather PNG image").Short('P').Default("false").Bool()
 	v2              = app.Flag("v2", "Use the v2 endpoint").Default("false").Bool()
 	transparency    = app.Flag("transparency", "Set transparency level (0-100) (PNG only)").Short('t').Default("0").Int()
-	location        = app.Flag("location", "Specify explicite location").Short('L').String()
-	language        = app.Flag("language", "Specify explicite language").Short('l').String()
+	location        = app.Flag("location", "Specify explicite location").Short('L').OverrideDefaultFromEnvar("GO_CMW_LOCATION").String()
+	language        = app.Flag("language", "Specify explicite language").Short('l').OverrideDefaultFromEnvar("GO_CMW_LANGUAGE").String()
 )
 
 // Create a function to parse all the command line parameters