Fix block chomping syntax in YAML lexer (#1234)

YAML supports several types of 'block chomping' syntax for block
strings. This commit lexes that syntax correctly and updates the visual
sample to include examples.

lib/rouge/lexers/yaml.rb

@@ -179,7 +179,7 @@ def set_indent(match, opts={})
         end
 
         # literal and folded scalars
-        rule %r/[\|>]/ do
+        rule %r/[\|>][+-]?/ do
           token Punctuation::Indicator
           push :block_scalar_content
           push :block_scalar_header

spec/visual/samples/yaml

@@ -146,6 +146,49 @@ stats: |
   65 Home Runs
   0.278 Batting Average
 
+# Multi-line block chomping
+
+## No newlines at end of folded block
+---
+desc: >-
+  This is just a dummy text intended to
+  test this lexer. The following whitespace
+  until "label" is intentional.
+
+
+label: lipsum
+
+## Allow all newlines at end of folded block
+---
+desc: >+
+  This is just a dummy text intended to
+  test this lexer. The following whitespace
+  until "label" is intentional.
+
+
+label: lipsum
+
+## No newlines at end of literal block
+---
+desc: |-
+  This is just a dummy text intended to
+  test this lexer. The following whitespace
+  until "label" is intentional.
+
+
+label: lipsum
+
+## Allow all newlines at end of literal block
+---
+desc: |+
+  This is just a dummy text intended to
+  test this lexer. The following whitespace
+  until "label" is intentional.
+
+
+label: lipsum
+...
+
 # Quoted scalars
 ---
 unicode: "Sosa did fine.\u263A"