allow all config variables to be set at top-level and overridden per-machine

All config variables now get initialized to UNSET_VALUE so that any per-machine variables that are unspecified can fall-through to a higher-level config when the config hierarchy is merged by vagrant.

Fixes #91.

lib/vagrant-hostmanager/config.rb

@@ -13,18 +13,22 @@ module VagrantPlugins
       alias_method :manage_host?, :manage_host
 
       def initialize
-        @enabled = false
+        @enabled            = UNSET_VALUE
         @manage_host        = UNSET_VALUE
         @ignore_private_ip  = UNSET_VALUE
         @include_offline    = UNSET_VALUE
-        @aliases = []
-        @ip_resolver = nil
+        @aliases            = UNSET_VALUE
+        @ip_resolver        = UNSET_VALUE
       end
 
       def finalize!
+        @enabled            = false if @enabled == UNSET_VALUE
         @manage_host        = false if @manage_host == UNSET_VALUE
         @ignore_private_ip  = false if @ignore_private_ip == UNSET_VALUE
         @include_offline    = false if @include_offline == UNSET_VALUE
+        @aliases            = [] if @aliases == UNSET_VALUE
+        @ip_resolver        = nil if @ip_resolver == UNSET_VALUE
+
         @aliases = [ @aliases ].flatten
       end