Added angle_brackets_escapable extension.

This is needed because github flavored Markdown has a slightly
different set of escapable symbols than original Markdown;
it includes angle brackets.

Closes #2846.

MANUAL.txt

@@ -3332,6 +3332,12 @@ in pandoc, but may be enabled by adding `+EXTENSION` to the format
 name, where `EXTENSION` is the name of the extension.  Thus, for
 example, `markdown+hard_line_breaks` is Markdown with hard line breaks.
 
+#### Extension: `angle_brackets_escapable` ####
+
+Allow `<` and `>` to be backslash-escaped, as they can be in
+GitHub flavored Markdown but not original Markdown.  This is
+implied by pandoc's default `all_symbols_escapable`.
+
 #### Extension: `lists_without_preceding_blankline` ####
 
 Allow a list to occur right after a paragraph, with no intervening
@@ -3475,7 +3481,7 @@ variants are supported:
 :   `pipe_tables`, `raw_html`, `fenced_code_blocks`, `auto_identifiers`,
     `ascii_identifiers`, `backtick_code_blocks`, `autolink_bare_uris`,
     `intraword_underscores`, `strikeout`, `hard_line_breaks`, `emoji`,
-    `shortcut_reference_links`.
+    `shortcut_reference_links`, `angle_brackets_escapable`.
 
 `markdown_mmd` (MultiMarkdown)
 :   `pipe_tables`, `raw_html`, `markdown_attribute`, `mmd_link_attributes`,

src/Text/Pandoc/Options.hs

@@ -101,6 +101,7 @@ data Extension =
                                -- space between items, and disallow laziness
     | Ext_example_lists       -- ^ Markdown-style numbered examples
     | Ext_all_symbols_escapable  -- ^ Make all non-alphanumerics escapable
+    | Ext_angle_brackets_escapable  -- ^ Make < and > escapable
     | Ext_intraword_underscores  -- ^ Treat underscore inside word as literal
     | Ext_blank_before_blockquote -- ^ Require blank line before a blockquote
     | Ext_blank_before_header     -- ^ Require blank line before a header
@@ -204,7 +205,8 @@ phpMarkdownExtraExtensions = Set.fromList
 
 githubMarkdownExtensions :: Set Extension
 githubMarkdownExtensions = Set.fromList
-  [ Ext_pipe_tables
+  [ Ext_angle_brackets_escapable
+  , Ext_pipe_tables
   , Ext_raw_html
   , Ext_fenced_code_blocks
   , Ext_auto_identifiers

src/Text/Pandoc/Readers/Markdown.hs

@@ -1507,6 +1507,8 @@ escapedChar' :: MarkdownParser Char
 escapedChar' = try $ do
   char '\\'
   (guardEnabled Ext_all_symbols_escapable >> satisfy (not . isAlphaNum))
+     <|> (guardEnabled Ext_angle_brackets_escapable >>
+            oneOf "\\`*_{}[]()>#+-.!~\"<>")
      <|> oneOf "\\`*_{}[]()>#+-.!~\""
 
 escapedChar :: MarkdownParser (F Inlines)