Support importing code snippets

Allowing to keep code in separate files, which can be cleaner in a
quite a few scenarios

lib/parse/code_include.js

@@ -0,0 +1,20 @@
+var fs = require('fs');
+var path = require('path');
+
+module.exports = function(code, folder) {
+    folder = folder || '';
+
+    return code.replace(/{{([\s\S]+?)}}/g, function(match, filename) {
+        // Normalize filename
+        var fname = path.join(folder, filename.trim());
+
+        // Try including snippet from FS
+        try {
+            // Trim trailing newlines/space of imported snippets
+            return fs.readFileSync(fname, 'utf8').trimRight();
+        } catch(err) {}
+
+        // If fails leave content as is
+        return match;
+    });
+};

lib/parse/renderer.js

@@ -1,6 +1,7 @@
 var url = require('url');
 var inherits = require('util').inherits;
 var links = require('../utils').links;
+var codeInclude = require('./code_include');
 
 
 var path = require('path');
@@ -125,7 +126,7 @@ GitBookRenderer.prototype._createCheckboxAndRadios = function(text) {
     var length = splittedText.length;
     var label = '<label class="quiz-label" for="' + quizIdentifier + '">' + splittedText[length - 1] + '</label>';
     return text.replace(fieldRegex, field).replace(splittedText[length - 1], label);
-}
+};
 
 GitBookRenderer.prototype.tablecell = function(content, flags) {
     return GitBookRenderer.super_.prototype.tablecell(this._createCheckboxAndRadios(content), flags);
@@ -135,5 +136,15 @@ GitBookRenderer.prototype.listitem = function(text) {
     return GitBookRenderer.super_.prototype.listitem(this._createCheckboxAndRadios(text));
 };
 
+GitBookRenderer.prototype.code = function(code, lang, escaped) {
+    return GitBookRenderer.super_.prototype.code.call(
+        this,
+        // Import code snippets
+        codeInclude(code, this._extra_options.dir),
+        lang,
+        escaped
+    );
+};
+
 // Exports
 module.exports = GitBookRenderer;