resolves #2106 make syntax highlighter pluggable (PR #3016)

- define the Asciidoctor::SyntaxHighlighter module to handle all aspects of syntax highlighting
- define built-in SyntaxHighlighter adapters for coderay, pygments, highlight.js, html-pipeline, and prettify
- move all embedded syntax highlighting logic into SyntaxHighlighter adapters
- allow SyntaxHighlighter adapter class to self register with a source highlighter name using register_for
- load SyntaxHighlighter adapters lazily (except for highlight.js and html-pipeline)
- add :syntax_highlighters and :syntax_highlighter_factory options to API to allow syntax highlighter resolution to be customized
- instantiate SyntaxHighlighter adapter once header is finalized if source-highlighter attribute is set on document and basebackend is html
- provide SyntaxHighlighter factory modules for registering syntax highlighter adapters (default, custom, and default proxy)
- allow SyntaxHighlighter adapters to contribute docinfo at :head and :footer slots in HTML output document
- move retrieval and writing of stylesheets for syntax highlighters to SyntaxHighlighter adapters
- wire methods in Stylesheets class that pertain to syntax highlighting to SyntaxHighlighter adapters
- remove hard-coded styles in Pygments output
- update default stylesheets to support Pygments styles and drop CSS workarounds
- don't fail if value of pygments-style attribute is not recognized; instead fallback to default style
- don't remove nested pre tags in HTML produced by Pygments when linenums are enabled
- update default stylesheet to avoid styling nested pre tags
- rename pygments table class to pygments-table
- change pygments base CSS selector from .listingblock .pygments to pre.pygments
- allow color for generic text, line numbers, and line number border to inherit from Pygments style
- align stylesheet for default CodeRay style with other syntax highlighters
- set default background color for prettify using a selector with the lowest precedence
- update and add tests
- add tests for the Pygments integration
- add pygments.rb dependency and enable Pygments integration tests if the environment variable PYGMENTS=true is set
- move syntax highlighter tests to dedicated test file

1 files changed, 4 insertions, 0 deletions

diff --git a/test/fixtures/source-block.adoc b/test/fixtures/source-block.adoc
new file mode 100644
index 00000000..0ab1a675
--- /dev/null
+++ b/test/fixtures/source-block.adoc
@@ -0,0 +1,4 @@
+[source,ruby]
+----
+puts 'Hello, World!'
+----