Fix #6: automatic section numbering.

Add chapter number to YAML, which is replace in numbering.css.
Use CSS counters for chapters, sections, subsections | examples.
Examples are detected by looking at the H3 element's id,
which redcarpet derives from the heading's title.
It must start with "Example:", and the whole title is suppressed
by a little javascript, so we can make Examples look like in the pdf.

For example, `### Example:` becomes `Example 3.2.10`,
if it's the 10th example in Section 3.2.

1 files changed, 56 insertions, 0 deletions

diff --git a/_includes/numbering.css b/_includes/numbering.css
new file mode 100644
index 0000000000..e8404652dc
--- /dev/null
+++ b/_includes/numbering.css
@@ -0,0 +1,56 @@
+// based on http://philarcher.org/css/numberheadings.css, 
+h1 {
+  /* must reset here */
+  counter-reset: chapter {{ page.chapter }};
+}
+h1:before {
+  /* and must reset again here */
+  counter-reset: chapter {{ page.chapter }};
+  content: "Chapter " counter(chapter);
+  display: block;
+}
+
+h2 {
+  /* must increment here */
+  counter-increment: section;
+  counter-reset: subsection;
+}
+h2:before {
+  /* and must reset again here */
+  counter-reset: chapter {{ page.chapter }};
+
+  content: counter(chapter) "." counter(section) ;
+  display: inline;
+  margin-right: 1em;
+}
+h2:after {
+  /* can only have one counter-reset per tag, so can't do it in h2/h2:before... */
+  counter-reset: example;
+}
+
+h3 {
+  /* must increment here */
+  counter-increment: subsection;
+}
+h3:before {
+  /* and must reset again here */
+  counter-reset: chapter {{ page.chapter }};
+
+  content: counter(chapter) "." counter(section) "." counter(subsection);
+  display: inline;
+  margin-right: 1em;
+}
+
+h3[id*='example:'] {
+  /* must increment here */
+  counter-increment: example;
+  display: inline;
+}
+h3[id*='example:']:before {
+  /* and must reset again here */
+  counter-reset: chapter {{ page.chapter }};
+
+  content: "Example " counter(chapter) "." counter(section) "." counter(example);
+  display: inline;
+  margin-right: 1em;
+}