fix(makeMarkdown): handle <br> tags converting html to markdown

Convert <br> tags to 2 spaces + newline rather than leaving the tag
in place when converting HTML to MD

src/subParsers/makemarkdown/break.js

@@ -0,0 +1,5 @@
+showdown.subParser('makeMarkdown.break', function () {
+  'use strict';
+
+  return '  \n';
+});

src/subParsers/makemarkdown/node.js

@@ -109,6 +109,10 @@ showdown.subParser('makeMarkdown.node', function (node, globals, spansOnly) {
       txt = showdown.subParser('makeMarkdown.image')(node, globals);
       break;
 
+    case 'br':
+      txt = showdown.subParser('makeMarkdown.break')(node, globals);
+      break;
+
     default:
       txt = node.outerHTML + '\n\n';
   }

test/functional/makemarkdown/cases/standard/breaks.html

@@ -0,0 +1 @@
+first line<br />and the second

test/functional/makemarkdown/cases/standard/breaks.md

@@ -0,0 +1,2 @@
+first line  
+and the second