Fix spacing issues when converting <strong>, <u>, <del>, <em> tags to markdown

[mumberrymountain]

fix issue #656

String html1 = "<p><strong>My strong text. </strong>test</p>";
String converted1= converter.convert(html1);
System.out.println(converted1); // **My strong text.**test

String html2 = "<p><strong>My strong text. </strong><span>test</span></p>";
String converted2= converter.convert(html2);
System.out.println(converted2); // **My strong text.** test

1. space wrapping for <strong>, <up>, <del>, <em> tags is applied only when next sibling is an element.

2. add a space before if the inner text starts with some special char (in 'prepend**.some**', bold is ignored because of the dot)