fix(strikethrough): allow escapinging tilde char

Since tilde is now used as a magic markdown character,
in strikethrough syntax, we need to enable escaping it.

Closes #331

dist/showdown.js

@@ -1644,7 +1644,7 @@ showdown.subParser('encodeAmpsAndAngles', function (text) {
 showdown.subParser('encodeBackslashEscapes', function (text) {
   'use strict';
   text = text.replace(/\\(\\)/g, showdown.helper.escapeCharactersCallback);
-  text = text.replace(/\\([`*_{}\[\]()>#+-.!])/g, showdown.helper.escapeCharactersCallback);
+  text = text.replace(/\\([`*_{}\[\]()>#+-.!~])/g, showdown.helper.escapeCharactersCallback);
   return text;
 });
 

src/subParsers/encodeBackslashEscapes.js

@@ -12,6 +12,6 @@
 showdown.subParser('encodeBackslashEscapes', function (text) {
   'use strict';
   text = text.replace(/\\(\\)/g, showdown.helper.escapeCharactersCallback);
-  text = text.replace(/\\([`*_{}\[\]()>#+-.!])/g, showdown.helper.escapeCharactersCallback);
+  text = text.replace(/\\([`*_{}\[\]()>#+-.!~])/g, showdown.helper.escapeCharactersCallback);
   return text;
 });

test/features/#331.allow-escaping-of-tilde.html

@@ -0,0 +1 @@
+<p>~~test~~</p>

test/features/#331.allow-escaping-of-tilde.md

@@ -0,0 +1 @@
+\~~test~~

test/node/testsuite.features.js

@@ -57,6 +57,8 @@ describe('makeHtml() features testsuite', function () {
       converter = new showdown.Converter({encodeEmails: false});
     } else if (testsuite[i].name === '#330.simplifiedAutoLink-drops-character-before-and-after-linked-mail') {
       converter = new showdown.Converter({encodeEmails: false, simplifiedAutoLink: true});
+    } else if (testsuite[i].name === '#331.allow-escaping-of-tilde') {
+      converter = new showdown.Converter({strikethrough: true});
     } else {
       converter = new showdown.Converter();
     }