'_' bug in parsing link and image

[breezewish]

/* Expected results are generated by marked in GFM mode. */

Hello! 2 new bugs :)

(1)

Input:

![Hello_world_img](/path/to/hello_world_img.jpg)

Output:

<img src="/path/to/hello_world_img.jpg" alt="Hello&lt;em&gt;world&lt;/em&gt;img">

Expected:

<img src="/path/to/hello_world_img.jpg" alt="Hello_world_img">

(2)

Input:

[Hello_world_link](/hello_world_link.html)

Output:

<a href="/hello&lt;em&gt;world&lt;/em&gt;link.html">Hello<em>world</em>link</a>

Expected:

<a href="/hello_world_link.html">Hello_world_link</a>

[kzykhys]

Thanks a lot!