feat(underline): add EXPERIMENTAL support for underline

Syntax is:
```
__double underscores__
or
___triple unserscores___
```
Keep in mind that, with this option enabled, underscore no longer
parses as `<em>` or `<strong>`

Closes #450

README.md

@@ -357,6 +357,9 @@ var defaultOptions = showdown.getDefaultOptions();
  * **emoji**: (boolean) [default false] Enable emoji support. Ex: `this is a :smile: emoji`
    For more info on available emojis, see https://github.com/showdownjs/showdown/wiki/Emojis **(since v.1.8.0)**
 
+ * **underline**: (boolean) [default false] ***EXPERIMENTAL FEATURE*** Enable support for underline.
+   Syntax is **double** or **triple** **underscores** ex: `__underlined word__`. With this option enabled, underscores are no longer parses into `<em>` and `<strong>`.
+
 **NOTE**: Please note that until **version 1.6.0**, all of these options are ***DISABLED*** by default in the cli tool.
  
 ## Flavors

src/options.js

@@ -145,6 +145,11 @@ function getDefaultOpts (simple) {
       defaultValue: false,
       description: 'Enable emoji support. Ex: `this is a :smile: emoji`',
       type: 'boolean'
+    },
+    underline: {
+      defaultValue: false,
+      description: 'Enable support for underline. Syntax is double or triple underscores: `__underline word__`. With this option enabled, underscores no longer parses into `<em>` and `<strong>`',
+      type: 'boolean'
     }
   };
   if (simple === false) {

src/subParsers/spanGamut.js

@@ -21,6 +21,7 @@ showdown.subParser('spanGamut', function (text, options, globals) {
   text = showdown.subParser('autoLinks')(text, options, globals);
   text = showdown.subParser('simplifiedAutoLinks')(text, options, globals);
   text = showdown.subParser('emoji')(text, options, globals);
+  text = showdown.subParser('underline')(text, options, globals);
   text = showdown.subParser('italicsAndBold')(text, options, globals);
   text = showdown.subParser('strikethrough')(text, options, globals);
 

src/subParsers/underline.js

@@ -0,0 +1,26 @@
+showdown.subParser('underline', function (text, options, globals) {
+  'use strict';
+
+  if (!options.underline) {
+    return text;
+  }
+
+  text = globals.converter._dispatch('underline.before', text, options, globals);
+
+  if (options.literalMidWordUnderscores) {
+    text = text.replace(/\b_?__(\S[\s\S]*)___?\b/g, function (wm, txt) {
+      return '<u>' + txt + '</u>';
+    });
+  } else {
+    text = text.replace(/_?__(\S[\s\S]*?)___?/g, function (wm, m) {
+      return (/\S$/.test(m)) ? '<u>' + m + '</u>' : wm;
+    });
+  }
+
+  // escape remaining underscores to prevent them being parsed by italic and bold
+  text = text.replace(/(_)/g, showdown.helper.escapeCharactersCallback);
+
+  text = globals.converter._dispatch('underline.after', text, options, globals);
+
+  return text;
+});