Give the user some way to control the text direction.

In the absence of a full BiDi algorithm, Balkón should support at least one way for increasing the embedding level, for example:

• by supporting the FSI and PDI characters in combination with LRM/RLM/ALM, which should work more-or-less automatically, or
• by supporting the unicode-bidi: isolate CSS property value in combination with direction: rtl or direction: ltr.

Requires #8.