feature: vpn-config-edit shows and changes the router's configuration.

README.md

@@ -240,6 +240,24 @@ vpn-router-setup` applies a changed configuration while Docker runs. With `docke
 `on`, the service fails when Docker is not installed or its daemon is not running; with
 `off` it removes the rules.
 
+## Editing the configuration
+
+`vpn-config-edit`, run as root, shows and changes `/etc/vpn-router/vpn-router.json`:
+
+```sh
+vpn-config-edit show
+vpn-config-edit set web.status_content standard
+vpn-config-edit set web.port 7082
+vpn-config-edit set web.trusted_proxies 127.0.0.1,10.0.0.4
+```
+
+`show` prints the file as indented JSON and does not change it. `set` takes a setting as
+`section.option` and converts the value to the setting's type: a whole number, `true` or
+`false`, a list separated by commas (an empty value is an empty list), or a string; an unknown
+setting is refused. It checks the whole configuration as `vpn-router-setup` does, and only then
+writes the file, indented, with mode 0600, and reminds that `vpn-router-setup` applies it. A
+refused request exits with status 2, any other failure with 1.
+
 ## Applying changes
 
 ```sh