Add plugin to create crumbs based on metadata

The `pretty_crumbs` plugin takes metadata information from the source
file (by default, the `crumbs` variable but this can be configured) and
uses that information for the breadcrum instead of the file/dirname.

Thanks to Chris Warrick for answering several questions I had as I
worked on this plugin.

v8/pretty_breadcrumbs/README.md

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+
+This plugin converts filepaths from breadcrumbs into more pretty
+breadcrumbs based on metadata information defined in the source
+file.
+
+For example, if file `test/index.rst` contains
+
+```
+.. crumb: This is a test
+```
+
+the breadcrumb generated would be `This is a test` instead of `test`.
+
+By default, the plugin looks for the metadata tag `crumb` but this
+can be changed via `PRETTY_BREADCRUMBS_TAG`.
+

v8/pretty_breadcrumbs/conf.py.sample

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+
+# By default, the pretty_crumbs plugin looks for the metadata tag `crumb`.
+# This can be changed with PRETTY_BREADCRUMBS_TAG.
+#PRETTY_BREADCRUMBS_TAG = "nav"
+

v8/pretty_breadcrumbs/pretty_breadcrumbs.plugin

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+[Core]
+Name = pretty_breadcrumbs
+Module = pretty_breadcrumbs
+
+[Nikola]
+PluginCategory = SignalHandler
+
+[Documentation]
+Author = Martin Michlmayr
+Version = 0.1
+Website = https://plugins.getnikola.com/#pretty_breadcrumbs
+Description = Generate pretty breadcrumbs based on metadata instead of filepaths
+

v8/pretty_breadcrumbs/pretty_breadcrumbs.py

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+# -*- coding: utf-8 -*-
+
+# Copyright © 2018 Martin Michlmayr
+
+# Permission is hereby granted, free of charge, to any
+# person obtaining a copy of this software and associated
+# documentation files (the "Software"), to deal in the
+# Software without restriction, including without limitation
+# the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the
+# Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice
+# shall be included in all copies or substantial portions of
+# the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
+# KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
+# WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+# PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS
+# OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+import blinker
+import os.path
+
+from nikola.plugin_categories import SignalHandler
+from nikola import utils
+
+
+class PrettyBreadcrumbs(SignalHandler):
+
+    def _set_pretty_crumbs(self, site):
+        old_get_crumbs = utils.get_crumbs
+
+        if 'PRETTY_BREADCRUMBS_TAG' in self.site.config:
+            tag = self.site.config['PRETTY_BREADCRUMBS_TAG']
+        else:
+            tag = 'crumb'
+
+        def pretty_get_crumbs(path, is_file=False, index_folder=None, lang=None):
+            lang = lang if lang else utils.LocaleBorg().current_lang
+            crumbs = old_get_crumbs(path, is_file=is_file, index_folder=index_folder, lang=lang)
+            _crumbs = []
+            for link, text in crumbs:
+                if link == '#':
+                    file = path
+                else:
+                    file = os.path.normpath(os.path.join(path, link))
+                if not is_file:
+                    file = os.path.join(file, self.site.config['INDEX_FILE'])
+                if file in self.site.post_per_file:
+                    post = self.site.post_per_file[file]
+                    if tag in post.meta[lang]:
+                        _crumbs.append([link, post.meta[lang][tag]])
+                        continue
+                _crumbs.append([link, text])
+            return _crumbs
+
+        # Set pretty_get_crumbs() as the new utils.get_crumbs()
+        utils.get_crumbs = pretty_get_crumbs
+
+    def set_site(self, site):
+        """Set site, which is a Nikola instance."""
+        super(PrettyBreadcrumbs, self).set_site(site)
+        # Add hook for after post scanning
+        blinker.signal("scanned").connect(self._set_pretty_crumbs)