Haml Listing Template v1.1.0

Fix `block_listing.html.haml`, the Haml HTML listing template used for
integrating Highlight into the Asciidoctor toolchain.

The original template was chewing up indentation of the first line in
code blocks that began with an indented line. This commit fixes the
problem by using the `:preserve` filter to ensure that indentation is
never affected. (Fixes #70)

_assets/hl/haml/block_listing.html.haml

@@ -1,7 +1,10 @@
--# "block_listing.html.haml"            v1.0.0 | 2019/03/01 | by Tristano Ajmone
+-# "block_listing.html.haml"            v1.1.0 | 2020/10/11 | by Tristano Ajmone
 -# =============================================================================
--#            Custom Haml template for Highlight syntax highlighter             
+-#            Custom Haml template for Highlight syntax highlighter
 -# =============================================================================
+-# Template tweaked to preserve first-line indentation via `:preserve` filter.
+-# The original was chewing up the indentation of the first line.
+-#
 -# Adds 'lang=<language>' to <pre> tag to allow using different CSS themes with
 -# each language. Example:
 -#
@@ -40,6 +43,8 @@
         - nowrap = false
       - pre_class << 'nowrap' if nowrap
       %pre{:class=>pre_class, :lang=>pre_lang}
-        %code{:class=>code_class, 'data-lang'=>code_lang}=content
+        %code{:class=>code_class, 'data-lang'=>code_lang}
+          :preserve
+            #{content}
     - else
       %pre{:class=>('nowrap' if nowrap)}=content