tools/dnsmasq-leases-html: create a HTML page with leases from dnsmasq

Parse the file that contains all the leases assigned by dnsmasq, and
create a static HTML page from it. This can be served by nginx to make
it easy to see what IP is assigned to a machine, and which machines are
currently on the network.

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+Generates a static HTML page with a list of all the leases allocated by `dnsmasq`.
+
+A simple template written in the jinja syntax is used.
+
+The file containing the leases is expected to be at `/var/lib/dnsmasq/dnsmasq.leases`, but this can be overwritten by setting the environment variable `DNSMASQ_LEASES`.
+
+The output of the script is written to `/var/lib/dnsmasq/leases.html` by default, but the destination can be overwritten by setting the environment variable `DNSMASQ_LEASES_OUT`.
+
+The script can be executed automatically by `dnsmasq` if the configuration for `dhcp-script` is set to the path of the script. This will only be executed when a *new* lease is created or an *old* lease is deleted. To execute the script when a lease is *updated* you need to use the configuration `script-on-renewal`.
+
+A configuration looks like this:
+
+``` ini
+dhcp-script=${pkgs.tools.dnsmasq-to-html}/bin/dnsmasq-leases-html
+script-on-renewal
+```
+
+## nginx
+To serve the page with nginx, you can use the following configuration:
+
+``` nix
+services.nginx = {
+  enable = true;
+  virtualHosts."dnsmasq" = {
+    listen = [
+      {
+        addr = "192.168.6.1";
+        port = 8067;
+      }
+    ];
+    locations."/" = {
+      root = "/var/lib/dnsmasq";
+      index = "leases.html";
+    };
+  };
+};
+```