Host specific env with tags

Allow hosts to be tagged so we can have host specific env variables.

We might want host specific env variables for things like datacenter
specific tags or testing GC settings on a specific host.

Right now you either need to set up a separate role, or have the app
be host aware.

Now you can define tag env variables and assign those to hosts.

For example:
```
servers:
  - 1.1.1.1
  - 1.1.1.2: tag1
  - 1.1.1.2: tag2
  - 1.1.1.3: [ tag1, tag2 ]
env_tags:
  tag1:
    ENV1: value1
  tag2:
    ENV2: value2
```

The tag env supports the full env format, allowing you to set secret and
clear values.

lib/kamal/configuration/env/tag.rb

@@ -0,0 +1,12 @@
+class Kamal::Configuration::Env::Tag
+  attr_reader :name, :config
+
+  def initialize(name, config:)
+    @name = name
+    @config = config
+  end
+
+  def env
+    Kamal::Configuration::Env.from_config(config: config)
+  end
+end

lib/kamal/configuration/role.rb

@@ -7,6 +7,7 @@ class Kamal::Configuration::Role
 
   def initialize(name, config:)
     @name, @config = name.inquiry, config
+    @tagged_hosts ||= extract_tagged_hosts_from_config
   end
 
   def primary_host
@@ -14,7 +15,11 @@ class Kamal::Configuration::Role
   end
 
   def hosts
-    @hosts ||= extract_hosts_from_config
+    tagged_hosts.keys
+  end
+
+  def env_tags(host)
+    tagged_hosts.fetch(host).collect { |tag| config.env_tag(tag) }
   end
 
   def cmd
@@ -50,12 +55,13 @@ class Kamal::Configuration::Role
   end
 
 
-  def env
-    @env ||= base_env.merge(specialized_env)
+  def env(host)
+    @envs ||= {}
+    @envs[host] ||= [ base_env, specialized_env, *env_tags(host).map(&:env) ].reduce(:merge)
   end
 
-  def env_args
-    env.args
+  def env_args(host)
+    env(host).args
   end
 
   def asset_volume_args
@@ -164,7 +170,24 @@ class Kamal::Configuration::Role
   end
 
   private
-    attr_accessor :config
+    attr_accessor :config, :tagged_hosts
+
+    def extract_tagged_hosts_from_config
+      {}.tap do |tagged_hosts|
+        extract_hosts_from_config.map do |host_config|
+          if host_config.is_a?(Hash)
+            raise ArgumentError, "Multiple hosts found: #{host_config.inspect}" unless host_config.size == 1
+
+            host, tags = host_config.first
+            tagged_hosts[host] = Array(tags)
+          elsif host_config.is_a?(String) || host_config.is_a?(Symbol)
+            tagged_hosts[host_config] = []
+          else
+            raise ArgumentError, "Invalid host config: #{host_config.inspect}"
+          end
+        end
+      end
+    end
 
     def extract_hosts_from_config
       if config.servers.is_a?(Array)