Include code that runs the chapter's project when selecting a chapter in code sandbox

Closes marijnh#89

Author: marijnh

04_data.txt

@@ -730,6 +730,7 @@ squirrel-lycanthropy. It _does_ seem to occur somewhat more often on
 weekends, however. Let's filter the results to show only correlations
 greater than 0.1 or less than -0.1.
 
+// start_code
 // test: no
 
 [source,javascript]

05_higher_order.txt

@@ -788,7 +788,9 @@ We can then use this to compute the amount of ((DNA)) my
 ((grandfather)) shared with Pauwels van Haverbeke, and divide that by
 four.
 
+// start_code bottom_lines: 2
 // test: clip
+// include_code top_lines: 6
 
 [source,javascript]
 ----

06_object.txt

@@ -956,6 +956,7 @@ them to our new prototype.
 function to use `RTextCell`'s for cells whose value is a number, we
 get the table we were aiming for:
 
+// start_code bottom_lines: 1
 // include_code strip_log
 
 [source,javascript]

07_elife.txt

@@ -1020,6 +1020,7 @@ var valley = new LifelikeWorld(
 
 ifdef::html_target[]
 
+// start_code
 // test: no
 
 [source,javascript]

11_language.txt

@@ -551,6 +551,8 @@ The function created by the `fun` form creates this local environment,
 and adds the argument variables to it. It then evaluates the function
 body in this environment, and returns the result.
 
+// start_code
+
 [source,javascript]
 ----
 run("do(define(plusOne, fun(a, +(a, 1))),",

15_game.txt

@@ -1152,6 +1152,8 @@ There is a set of level plans available in the `GAME_LEVELS` variable
 http://eloquentjavascript.net/code[_eloquentjavascript.net/code_])!).
 This page feeds them to `runGame`, starting an actual game:
 
+// start_code
+
 [sandbox="null"]
 [focus="yes"]
 [source,text/html]

16_canvas.txt

@@ -1041,6 +1041,8 @@ ifdef::html_target[]
 
 The tiny document below plugs the new display into `runGame`:
 
+// start_code
+
 [sandbox="game"]
 [focus="yes"]
 [source,text/html]

18_forms.txt

@@ -605,6 +605,8 @@ because only strings can be stored there. The user can select a note
 from a `<select>` field, and change that note's text in a
 `<textarea>`. A note can be added by pressing a button.
 
+// start_code
+
 [source,text/html]
 ----
 Notes: <select id="list"></select>

19_paint.txt

@@ -623,6 +623,8 @@ try it out.!)
 
 ifdef::html_target[]
 
+// start_code
+
 [sandbox="paint"]
 [source,text/html]
 ----

bin/build_code.js

@@ -3,7 +3,7 @@ var fs = require("fs");
 var file = process.argv[2];
 var input = fs.readFileSync(file, "utf8");
 
-var included = /\/\/ include_code(.*)\n(?:\/\/.*\n)*\s*(?:\[sandbox=.*\n|\[\[.*\n)*\[source,.*?\]\n----\n([\s\S]*?\n)----/g, m;
+var included = /\/\/ include_code(.*)\n(?:\/\/.*\n)*\s*(?:\[.*\n)*\[source,.*?\]\n----\n([\s\S]*?\n)----/g, m;
 var files = Object.create(null);
 var defaultFile = "code/chapter/" + file.replace(".txt", ".js");