Don't trim non-ASCII whitespace

[tats-u]

Fixes #261

.trim() removes non-ascii Unicode white spaces as well.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trim

PS D:\cmark\out\install\x64-Debug\bin> .\cmark.exe
　全角スペース (U+3000) 全形空白　

 Em Space (U+2003) Em Space 

 NBSP (U+00A0) NBSP
^Z
<p>　全角スペース (U+3000) 全形空白　</p>
<p>?Em Space (U+2003) Em Space?</p>
<p>NBSP (U+00A0) NBSP</p>

cmark removes NBSP, but is this an expected behavior?
(? means U+2003 can't be converted to Shift_JIS, which is the OEM encoding in Japanese Windows.)