Add more syntax highlighting to README (#937)

Author: alex-tan

README.md

@@ -22,55 +22,63 @@ MacRuby and RubyMotion support sponsored by [CodeClimate](http://codeclimate.com
 Load Parser (see the [backwards compatibility](#backwards-compatibility) section
 below for explanation of `emit_*` calls):
 
-    require 'parser/current'
-    # opt-in to most recent AST format:
-    Parser::Builders::Default.emit_lambda              = true
-    Parser::Builders::Default.emit_procarg0            = true
-    Parser::Builders::Default.emit_encoding            = true
-    Parser::Builders::Default.emit_index               = true
-    Parser::Builders::Default.emit_arg_inside_procarg0 = true
-    Parser::Builders::Default.emit_forward_arg         = true
-    Parser::Builders::Default.emit_kwargs              = true
-    Parser::Builders::Default.emit_match_pattern       = true
+```ruby
+require 'parser/current'
+# opt-in to most recent AST format:
+Parser::Builders::Default.emit_lambda              = true
+Parser::Builders::Default.emit_procarg0            = true
+Parser::Builders::Default.emit_encoding            = true
+Parser::Builders::Default.emit_index               = true
+Parser::Builders::Default.emit_arg_inside_procarg0 = true
+Parser::Builders::Default.emit_forward_arg         = true
+Parser::Builders::Default.emit_kwargs              = true
+Parser::Builders::Default.emit_match_pattern       = true
+```
 
 Parse a chunk of code:
 
-    p Parser::CurrentRuby.parse("2 + 2")
-    # (send
-    #   (int 2) :+
-    #   (int 2))
+```ruby
+p Parser::CurrentRuby.parse("2 + 2")
+# (send
+#   (int 2) :+
+#   (int 2))
+```
 
 Access the AST's source map:
 
-    p Parser::CurrentRuby.parse("2 + 2").loc
-    # #<Parser::Source::Map::Send:0x007fe5a1ac2388
-    #   @dot=nil,
-    #   @begin=nil,
-    #   @end=nil,
-    #   @selector=#<Source::Range (string) 2...3>,
-    #   @expression=#<Source::Range (string) 0...5>>
-
-    p Parser::CurrentRuby.parse("2 + 2").loc.selector.source
-    # "+"
+```ruby
+p Parser::CurrentRuby.parse("2 + 2").loc
+# #<Parser::Source::Map::Send:0x007fe5a1ac2388
+#   @dot=nil,
+#   @begin=nil,
+#   @end=nil,
+#   @selector=#<Source::Range (string) 2...3>,
+#   @expression=#<Source::Range (string) 0...5>>
+
+p Parser::CurrentRuby.parse("2 + 2").loc.selector.source
+# "+"
+```
 
 Traverse the AST: see the documentation for [gem ast](https://whitequark.github.io/ast/).
 
 Parse a chunk of code and display all diagnostics:
 
-    parser = Parser::CurrentRuby.new
-    parser.diagnostics.consumer = lambda do |diag|
-      puts diag.render
-    end
-
-    buffer = Parser::Source::Buffer.new('(string)', source: "foo *bar")
-
-    p parser.parse(buffer)
-    # (string):1:5: warning: `*' interpreted as argument prefix
-    # foo *bar
-    #     ^
-    # (send nil :foo
-    #   (splat
-    #     (send nil :bar)))
+```ruby
+parser = Parser::CurrentRuby.new
+parser.diagnostics.consumer = lambda do |diag|
+  puts diag.render
+end
+
+buffer = Parser::Source::Buffer.new('(string)', source: "foo *bar")
+
+p parser.parse(buffer)
+# (string):1:5: warning: `*' interpreted as argument prefix
+# foo *bar
+#     ^
+# (send nil :foo
+#   (splat
+#     (send nil :bar)))
+```
 
 If you reuse the same parser object for multiple `#parse` runs, you need to
 `#reset` it.
@@ -138,7 +146,7 @@ Several Parser nodes seem to be confusing enough to warrant a dedicated README s
 
 The `(block)` node passes a Ruby block, that is, a closure, to a method call represented by its first child, a `(send)`, `(super)` or `(zsuper)` node. To demonstrate:
 
-```
+```bash
 $ ruby-parse -e 'foo { |x| x + 2 }'
 (block
   (send nil :foo)
@@ -162,7 +170,7 @@ Both `(begin)` and `(kwbegin)` nodes represent compound statements, that is, sev
 
 and so on.
 
-```
+```bash
 $ ruby-parse -e '(foo; bar)'
 (begin
   (send nil :foo)