Add config_role plugin

v7/config_role/README.md

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+Add the `config` role, it reads a value from the `conf.py` file.
+
+Usage:
+
+```rst
+Send us an email to :config:`BLOG_EMAIL`
+```
+
+Here `BLOG_EMAIL` is the variable name from the `conf.py` file.
+Note that the text is rendered as an email link, not just as plain text.
+This is because the value is interpreted as reStructuredText.

v7/config_role/config_role.plugin

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+[Core]
+Name = config_role
+Module = config_role
+
+[Nikola]
+Compiler = rest
+PluginCategory = RestExtension
+
+[Documentation]
+Author = Santos Gallegos
+Version = 0.1
+Website = http://plugins.getnikola.com/#config_role
+Description = Inserts values from config.py

v7/config_role/config_role.py

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+"""
+Add the ``config`` role, it reads a value from the `conf.py` file.
+
+Usage:
+
+   Send us an email to :config:`BLOG_EMAIL`
+
+Here `BLOG_EMAIL` is the variable name from the conf.py file.
+Note that the text is rendered as an email link, not just as plain text.
+This is because the value is interpreted as reStructuredText.
+"""
+
+from docutils.frontend import OptionParser
+from docutils.parsers.rst import Parser, roles
+from docutils.utils import new_document
+from nikola.plugin_categories import RestExtension
+
+
+class Plugin(RestExtension):
+
+    name = 'config_role'
+
+    def set_site(self, site):
+        roles.register_canonical_role('config', config_role(site))
+        return super().set_site(site)
+
+
+def config_role(site):
+    """
+    Inject a value from the ``conf.py`` file.
+
+    Before injecting the value, it is parsed by the rst parser,
+    that way we can have a link when reading an email from the config
+    for example.
+
+    This role needs access to the `site` object.
+    As we can't pass extra args to a role function we are using a closure.
+    """
+    def role(name, rawtext, text, lineno, inliner, options=None, content=None):
+        parser = Parser()
+        settings = OptionParser(components=(Parser,)).get_default_values()
+        document = new_document('Raw rst from config value', settings)
+
+        config_value = site.config.get(text, '')
+        parser.parse(config_value, document)
+
+        # The whole text is interpreted as a big paragraph
+        # causing a new line to be inserted at the end,
+        # so we only take the elements inside that paragraph.
+        paragraph, *_ = document.children
+        return paragraph.children, []
+    return role