Adds an optional fix for right-to-left text

[AlonTzarafi]

Rtl text was disordering the calendar cells between columns. Added a
setting to enable a fix.

The fix is made optional because it depends on a special unicode character (the "LEFT-TO-RIGHT EMBEDDING" character).

[kiwanami]

I'm sorry for my slow response.
Thank you for your PR!
Your code looks good for me, but I don't know how to check it because I have not used r2l function on emacs.
Will you tell me how to check?

[AlonTzarafi]

Hi kiwanami. Thanks for checking out the PR.
The easiest way to see the problem is having a Hebrew calendar event and an English one after it.
Modern Emacs will display the Hebrew words, but the order will be garbled in the calendar view.
You can try it by launching the monthly calendar view, to display these two org-mode events:

** אירוע בעברית!
   SCHEDULED: <2017-12-13 Wed 12:00>
** English event!
   SCHEDULED: <2017-12-14 Thu 12:00>

Then, set the fix:
(setq cfw:fix-rtl-content t)
And refresh the calendar buffer.

[kiwanami]

Hi @AlonTzarafi
Thank you for advice on how to check.
I'll try soon.