allow to pass the token via env var GOTIFY_TOKEN

Passing the token as an argument has the implication that the token will
show up in the process list.
To solve this issue an environment variable `GOTIFY_TOKEN` can be used
to pass the token to gotify.

Example:

```
GOTIFY_TOKEN=app_token_here
MSG="Test message"

gotify push $MSG
```

README.md

@@ -141,7 +141,7 @@ OPTIONS:
    --priority value, -p value  Set the priority (default: 0)
    --exec value, -x value      Pass command to exec (default: "sh -c")
    --title value, -t value     Set the title (empty for command)
-   --token value               Override the app token
+   --token value               Override the app token [$GOTIFY_TOKEN]
    --url value                 Override the Gotify URL
    --output value, -o value    Output verbosity (short|default|long) (default: "default")
 ```
@@ -163,7 +163,7 @@ DESCRIPTION:
 OPTIONS:
    --priority value, -p value    Set the priority (default: 0)
    --title value, -t value       Set the title (empty for app name)
-   --token value                 Override the app token
+   --token value                 Override the app token [$GOTIFY_TOKEN]
    --url value                   Override the Gotify URL
    --quiet, -q                   Do not output anything (on success)
    --contentType value           The content type of the message. See https://gotify.net/docs/msgextras#client-display

command/push.go

@@ -25,7 +25,7 @@ func Push() cli.Command {
 		Flags: []cli.Flag{
 			cli.IntFlag{Name: "priority,p", Usage: "Set the priority"},
 			cli.StringFlag{Name: "title,t", Usage: "Set the title (empty for app name)"},
-			cli.StringFlag{Name: "token", Usage: "Override the app token"},
+			cli.StringFlag{Name: "token", Usage: "Override the app token", EnvVar: "GOTIFY_TOKEN"},
 			cli.StringFlag{Name: "url", Usage: "Override the Gotify URL"},
 			cli.BoolFlag{Name: "quiet,q", Usage: "Do not output anything (on success)"},
 			cli.StringFlag{Name: "contentType", Usage: "The content type of the message. See https://gotify.net/docs/msgextras#client-display"},

command/watch.go

@@ -25,7 +25,7 @@ func Watch() cli.Command {
 			cli.IntFlag{Name: "priority,p", Usage: "Set the priority"},
 			cli.StringFlag{Name: "exec,x", Usage: "Pass command to exec", Value: "sh -c"},
 			cli.StringFlag{Name: "title,t", Usage: "Set the title (empty for command)"},
-			cli.StringFlag{Name: "token", Usage: "Override the app token"},
+			cli.StringFlag{Name: "token", Usage: "Override the app token", EnvVar: "GOTIFY_TOKEN"},
 			cli.StringFlag{Name: "url", Usage: "Override the Gotify URL"},
 			cli.StringFlag{Name: "output,o", Usage: "Output verbosity (short|default|long)", Value: "default"},
 		},