Do not emit newline after <img> tag.

[dmitshur]

• This changes HTML renderer not to always add a newline character after tags. This is desirable because tags can be inlined, and sometimes you want to avoid whitespace on left and right sides. Previous behavior of always adding a newline would unavoidably create whitespace after tag.
• Update all tests to match new behavior. There are few changes, and they're completely isolated to inline image tests.

Fixes #169.

Change Overview

Consider the following Markdown input:

Title
===

An inline image ![img](http://www.example.com/image.png) looks like that.

![standalone image](http://www.example.com/image.png)

Another paragraph.

Previously, this would generate the following HTML output:

<h1>Title</h1>

<p>An inline image <img src="http://www.example.com/image.png" alt="img" />
 looks like that.</p>

<p><img src="http://www.example.com/image.png" alt="standalone image" />
</p>

<p>Another paragraph.</p>

After this PR, it will generate this instead:

<h1>Title</h1>

<p>An inline image <img src="http://www.example.com/image.png" alt="img" /> looks like that.</p>

<p><img src="http://www.example.com/image.png" alt="standalone image" /></p>

<p>Another paragraph.</p>

That looks like an improvement in rendering to me, and I think this change should be done.

[xyproto]

Tested the no-newline-after-img branch. It solved the problem here. The pull request looks good.

[rtfb]

Great, thanks, Dmitri!

[dmitshur]

Thanks for reviewing and merging!