Allow passing options to SSH command in rsync mode

The same method you use to pass flags to the rsync command itself whould
be useful to the ssh command passed to rsync as the `-e ssh` flag.

README.md

@@ -30,10 +30,11 @@ activate :deploy do |deploy|
   deploy.host          = 'www.example.com'
   deploy.path          = '/srv/www/site'
   # Optional Settings
-  # deploy.user  = 'tvaughan' # no default
-  # deploy.port  = 5309 # ssh port, default: 22
-  # deploy.clean = true # remove orphaned files on remote host, default: false
-  # deploy.flags = '-rltgoDvzO --no-p --del' # add custom flags, default: -avz
+  # deploy.user      = 'tvaughan' # no default
+  # deploy.port      = 5309 # ssh port, default: 22
+  # deploy.clean     = true # remove orphaned files on remote host, default: false
+  # deploy.flags     = '-rltgoDvzO --no-p --del' # add custom flags, default: -avz
+  # deploy.ssh_flags = '-o UserKnownHostsFile=~/.ssh/known_hosts' # add custom ssh flags, no default
 end
 ```
 

lib/middleman-deploy/extension.rb

@@ -26,6 +26,7 @@ class Extension < Extension
       option :build_before, nil
       option :flags, nil
       option :commit_message, nil
+      option :ssh_flags, nil
 
       def initialize(app, options_hash = {}, &block)
         super

lib/middleman-deploy/methods/rsync.rb

@@ -7,12 +7,13 @@ class Rsync < Base
         def initialize(server_instance, options = {})
           super(server_instance, options)
 
-          @clean  = self.options.clean
-          @flags  = self.options.flags
-          @host   = self.options.host
-          @path   = self.options.path
-          @port   = self.options.port
-          @user   = self.options.user
+          @clean     = self.options.clean
+          @flags     = self.options.flags
+          @host      = self.options.host
+          @path      = self.options.path
+          @port      = self.options.port
+          @user      = self.options.user
+          @ssh_flags = self.options.ssh_flags
         end
 
         def process
@@ -21,7 +22,7 @@ def process
 
           dest_url  = "#{user}#{host}:#{path}"
           flags     = self.flags || '-avz'
-          command   = "rsync #{flags} '-e ssh -p #{port}' #{build_dir}/ #{dest_url}"
+          command   = "rsync #{flags} '-e ssh -p #{port} #{@ssh_flags}' #{build_dir}/ #{dest_url}"
 
           command += ' --delete' if clean