Require a space after : for definition list

[litao91]

Hello
:Word 

Will cause panic

[yuin]

Thanks for your PR.
"Space" in CommonMark is not only spaces but tabs.

term
:[space][space][space]definition1
[space][space][space][space]definition2

->

<dl>
<dt>term</dt>
<dd>definition
definition2</dd>
</dl>

term
:[space][space][space]definition1
[space][space][space][space][space]definition2

->

<dl>
<dt>term</dt>
<dd>definition
[space]definition2</dd>
</dl>

term
:[TAB]definition1
[space][space][space][space]definition2

->

<dl>
<dt>term</dt>
<dd>definition
definition2</dd>
</dl>

term
:[TAB][TAB]definition1
[space][space][space][space][space][space][space][space]definition2

->

<dl>
<dt>term</dt>
<dd><pre><code>definition1
definition2
</code></pre></dd>
</dl>

If you feel hard to fix the issue, could you submit a issue to this project?
I'll fix the issue as quick as I can.

[yuin]

I've fixed this bug while fixing bugs found in fuzzing. Thanks for your efforts!