Combination (#578)

This PR combines (and resolves conflicts between) #448, #463, #466, #494, #495, #496, #498, and #572. 

The main aim is to facilitate use of several of the implemented features _together_, when using the fork as a remote theme. It should also simplify merging the included PRs into a future release.

The branch [combination-rec-nav](https://github.com/pdmosses/just-the-docs/tree/combination-rec-nav) adds [multi-level navigation](https://github.com/pmarsceill/just-the-docs/pull/462) and (NEW:) [sibling links](https://github.com/pmarsceill/just-the-docs/pull/394) to the branch used for this PR. It includes updated [documentation for the navigation structure](https://pdmosses.github.io/just-the-docs/docs/navigation-structure/), and reorganised and extended [navigation tests](https://pdmosses.github.io/just-the-docs/tests/navigation/). The documentation and the tests can be browsed at the (temporary) [website published from the combination-rec-nav branch](https://pdmosses.github.io/just-the-docs/).

_Caveat:_ The changes to v0.3.3 in this PR and #462 have not yet been reviewed or approved, and may need updating before merging into a release of the theme. If you use a branch from a PR as a remote theme, there is a risk of such updates affecting your website. Moreover, these branches are likely to be deleted after they have been merged. To avoid  such problems, you could copy the branch that you want to use to your own fork of the theme.

Co-authored-by: Matt Wang <matt@matthewwang.me>

docs/index-test.md

@@ -4,6 +4,22 @@ title: Markdown kitchen sink
 nav_order: 99
 ---
 
+<button class="btn js-toggle-dark-mode">Preview dark color scheme</button>
+
+<script>
+const toggleDarkMode = document.querySelector('.js-toggle-dark-mode');
+
+jtd.addEvent(toggleDarkMode, 'click', function(){
+  if (jtd.getTheme() === 'dark') {
+    jtd.setTheme('light');
+    toggleDarkMode.textContent = 'Preview dark color scheme';
+  } else {
+    jtd.setTheme('dark');
+    toggleDarkMode.textContent = 'Return to the light side';
+  }
+});
+</script>
+
 Text can be **bold**, _italic_, or ~~strikethrough~~.
 
 [Link to another page](another-page).
@@ -172,6 +188,41 @@ Term2
 
 ### More code
 
+```python{% raw %}
+def dump_args(func):
+    "This decorator dumps out the arguments passed to a function before calling it"
+    argnames = func.func_code.co_varnames[:func.func_code.co_argcount]
+    fname = func.func_name
+    def echo_func(*args,**kwargs):
+        print fname, ":", ', '.join(
+            '%s=%r' % entry
+            for entry in zip(argnames,args) + kwargs.items())
+        return func(*args, **kwargs)
+    return echo_func
+
+@dump_args
+def f1(a,b,c):
+    print a + b + c
+
+f1(1, 2, 3)
+
+def precondition(precondition, use_conditions=DEFAULT_ON):
+    return conditions(precondition, None, use_conditions)
+
+def postcondition(postcondition, use_conditions=DEFAULT_ON):
+    return conditions(None, postcondition, use_conditions)
+
+class conditions(object):
+    __slots__ = ('__precondition', '__postcondition')
+
+    def __init__(self, pre, post, use_conditions=DEFAULT_ON):
+        if not use_conditions:
+            pre, post = None, None
+
+        self.__precondition  = pre
+        self.__postcondition = post
+{% endraw %}```
+
 ```
 Long, single-line code blocks should not wrap. They should horizontally scroll if they are too long. This line should be long enough to demonstrate this.
 ```