Adds support for the ``.. code::`` rst directive.

This modifies the existing ``.. code-block::`` to be treated as spec
extension with a specific default language syntax. Also implements line
numbering.
Modifies docgen to support new warning message.
Improves macro tutorial with hyperlinks and numbered lines.
Hyperlinks quit related docstrings in system module.
Hyperlinks slurp related docstrings in system module.
Handles :file: option for code blocks to avoid warnings.
Fixes line numbering.
Merge branch 'pr_adds_code_directive_to_rst' into pr_bigbreak_adds_code_directive_to_rst

Conflicts:
	doc/tut2.txt
	lib/packages/docutils/rstgen.nim
	lib/system.nim
Merge pull request #1623 from gradha/pr_bigbreak_adds_code_directive_to_rst

Adds bigbreak code directive to rst with numbered lines