Handle ctrl+c in repl, add exit function

bin/chervil

 require "chervil"
 require "readline"
 
+def repl(env)
+  begin
+    while line = Readline.readline("> ", true)
+      break if line == "(exit)"
+      lexer = Chervil::Lexer.new(line)
+      tree = Chervil::Parser.new(lexer).parse
+      results = Chervil::Interpreter.new(tree, env).interpret
+      results.each { |result| puts result unless result.nil? }
+    end
+  rescue Interrupt
+    puts
+    repl(env)
+  end
+end
 
 env = Chervil::Env.new
-while line = Readline.readline("> ", true)
-  lexer = Chervil::Lexer.new(line)
-  tree = Chervil::Parser.new(lexer).parse
-  results = Chervil::Interpreter.new(tree, env).interpret
-  results.each { |result| puts result unless result.nil? }
-end
+repl(env)