Allow RTL formatting in markdown mode

[belcherj]

Fix

Fixes: #562

Allow RTL languages to be right aligned in markdown preview mode.

Before:

After:

Test

1. Have several notes with markdown
2. Does markdown look properly in preview mode

[dmsnell]

This tested fine in my test document with mixed-direction content in separate paragraphs but also inline. I don't understand the implications of unicode-bidi property but plaintext seems reasonable.

[belcherj]

https://developer.mozilla.org/en-US/docs/Web/CSS/unicode-bidi

plaintext
This keyword makes the elements directionality calculated without considering its parent bidirectional state or the value of the direction property. The directionality is calculated using the P2 and P3 rules of the Unicode Bidirectional Algorithm.
This value allows the display of data that is already formatted using a tool following the Unicode Bidirectional Algorithm.