markdown: Add support for quoted text

[machour]

When replying from an email, the notification content is embedded in the reply.

On github.com, it's displayed as a foldable section, folded by default:

Currently the app doesn't take care of it:

Should be handled in the same way.

[andrewda]

Maybe we could solve this with some regex-fu.

(?:\.\.\.|…)[\s\S]+Reply to this email directly, view it on GitHub[\s\S]+

Not sure if that's an ellipsis character (…) or three periods (...), so just choose the appropriate one for the first group.

[machour]

@andrewda FYI, Github HTML is already pre-processed like this:

<div class="email-hidden-toggle"><a href="#">…</a></div>
<div class="email-hidden-reply" style="display:none">
<div class="email-quoted-reply">On Oct 8, 2017 5:52 AM, "Alexey Pyltsyn" ***@***.***&gt; wrote:
 <a href="https://github.com/SammyIsra" class="user-mention">@SammyIsra</a> &lt;<a href="https://github.com/sammyisra">https://github.com/sammyisra</a>&gt; I looked at the emulator with
 Android 7.1.1 and a screen of 5.5 inches (like One Plus 3T), but there is
 no such problem. Is this a new version of the application?

 —
 You are receiving this because you were mentioned.
 Reply to this email directly, view it on GitHub
 &lt;<a href="https://github.com/gitpoint/git-point/issues/433#issuecomment-334995398" class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="263313028" data-permission-text="Issue title is private" data-url="https://github.com/gitpoint/git-point/issues/433">#433 (comment)</a>&gt;,
 or mute the thread
 &lt;<a href="https://github.com/notifications/unsubscribe-auth/AIjAtIKarjxNjv7JpoYKiSwZ_uGnDBHPks5sqJt6gaJpZM4Pv52h">https://github.com/notifications/unsubscribe-auth/AIjAtIKarjxNjv7JpoYKiSwZ_uGnDBHPks5sqJt6gaJpZM4Pv52h</a>&gt;
 .
</div>
<div class="email-fragment"></div>
</div>