Add support for basic description/definition lists

[hisaac]

I find myself using descriptions lists, so I've added support for them in my Hexo setup, and want to offer it here to be included in the main repo. This will turn a PHP Markdown style definition list into the correct definition list html.

So this Markdown:

Definition Term
:    This is the definition for the term

Will generate this html:

<dl>
  <dt>Definition Term</dt>
  <dd>This is the definition for the term</dd>
</dl>

There is a limitation in the <dd>. This only supports single definitions. The above Markdown will work, but this will not:

Definition Term
:    Definition 1
:    Definition 2

That Markdown will generate this html:

<dl>
  <dt>Definition Term<br>: Definition 1</dt>
  <dd>Definition 2</dd>
</dl>

If you've got ideas on how to support multiple definitions, I'd love to add that.

[coveralls]

Coverage increased (+1.4%) to 88.679% when pulling ab96602 on hisaac:master into 291b47a on hexojs:master.

[coveralls]

Coverage increased (+1.9%) to 89.091% when pulling 03c790f on hisaac:master into 291b47a on hexojs:master.

[NoahDragon]

Thanks for the sharing. Really cool feature. Could you please also update the Readme file to reflect this change?

[hisaac]

Definitely. Documentation is important. Let me know what you think of the additions I made.

[coveralls]

Coverage increased (+1.9%) to 89.091% when pulling bd87b06 on hisaac:master into 291b47a on hexojs:master.

[coveralls]

Coverage increased (+1.9%) to 89.091% when pulling bd87b06 on hisaac:master into 291b47a on hexojs:master.

[NoahDragon]

Great job. Thank you!