Allow use of Pygments to colorize source code

Allow use of Pygments to colorize source code

While GNU Source Highlight is good, it's also difficult to build and
distribute.  For one thing, it needs Boost.  For another, it has an
unusual configuration and installation setup.

Pygments, a Python library, doesn't suffer from these issues, and so I
thought it would be a reasonable fallback.

This patch implements this idea.  GNU Source Highlight is preferred,
but if it is unavailable (or fails), the extension languages are
tried.  This patch also implements support for Pygments.

Something similar could be done for Guile, using:

    https://dthompson.us/projects/guile-syntax-highlight.html

However, I don't know enough about Guile internals to make this
happen, so I have not done it here.

gdb/ChangeLog
2020-01-21  Tom Tromey  <tromey@adacore.com>

	* source-cache.c (source_cache::ensure): Call ext_lang_colorize.
	* python/python.c (python_extension_ops): Update.
	(gdbpy_colorize): New function.
	* python/lib/gdb/__init__.py (colorize): New function.
	* extension.h (ext_lang_colorize): Declare.
	* extension.c (ext_lang_colorize): New function.
	* extension-priv.h (struct extension_language_ops) <colorize>: New
	member.
	* cli/cli-style.c (_initialize_cli_style): Update help text.

Change-Id: I5e21623ee05f1f66baaa6deaeca78b578c031bf4