[BREAKING] Add RTL support

[cibernox]

Attack plan:

• Rename "left" to "start" and "right" to "end" in positioning strategies.
• Detect if the page is rtl or ltr (checking the dir attribute on the html) and make "start" and "end" flip their calculation based on that.
• By default the positioning strategy will be "auto-start" (the same way right now is "auto-left"), but we'll add a "reversed" strategy to make it be "auto-end" in ltr pages and "auto-start".