Yield the last created flow on Logger.new

This allows to configure the last (and often only) flow on creation of
the Logger without having to manually create the object. We can thus use
the following shortcut to e.g. set a custom encoder:

    Rackstash::Logger.new(STDOUT) do
      encoder Rackstash::Encoder::Message.new
    end

lib/rackstash/logger.rb

@@ -57,13 +57,24 @@ module Rackstash
     #   By default we use {PROGNAME}.
     # @param formatter [#call] the log formatter for each individual buffered
     #   line. See {#formatter} for details.
-    def initialize(*flows, level: DEBUG, progname: PROGNAME, formatter: Formatter.new)
+    # @yieldparam flow [Rackstash::Flow] if the given block accepts an argument,
+    #   we yield the last {Flow} as a parameter. Without an expected argument,
+    #   the block is directly executed in the context of the last {Flow}.
+    def initialize(*flows, level: DEBUG, progname: PROGNAME, formatter: Formatter.new, &block)
       @buffer_stack = Concurrent::ThreadLocalVar.new
 
       @sink = Rackstash::Sink.new(*flows)
       self.level = level
       self.progname = progname
       self.formatter = formatter
+
+      if block_given? && (flow = @sink.flows.last)
+        if block.arity == 0
+          flow.instance_eval(&block)
+        else
+          yield flow
+        end
+      end
     end
 
     # Add a message to the current buffer without any further formatting. If

spec/rackstash/logger_spec.rb

@@ -35,6 +35,35 @@ describe Rackstash::Logger do
       logger = described_class.new('output.log', formatter: -> {})
       expect(logger.formatter).to be_a Proc
     end
+
+    it 'yields the last flow to a parameterized block' do
+      block_called = 0
+      block_self = nil
+      block_args = nil
+
+      described_class.new(StringIO.new, StringIO.new) do |*args|
+        block_called += 1
+        block_self = self
+        block_args = args
+      end
+
+      expect(block_called).to eql 1
+      expect(block_self).to equal self
+      expect(block_args).to match [instance_of(Rackstash::Flow)]
+    end
+
+    it 'instance_evals the parameter-less block in the last flow' do
+      block_called = 0
+      block_self = nil
+
+      described_class.new(StringIO.new, StringIO.new) do
+        block_called += 1
+        block_self = self
+      end
+
+      expect(block_called).to eql 1
+      expect(block_self).to be_instance_of(Rackstash::Flow)
+    end
   end
 
   describe 'subscript accessors' do