Support RTL languages

Downstream issue: https://github.com/Microsoft/vscode/issues/28571

When we enforced unicode character width in https://github.com/sourcelair/xterm.js/issues/467 this broke RTL language characters as they are now rendered in reverse (LTR). We could revert that for RTL character ranges only but we should do the right fix and reverse the strings so they're actually on the character grid as the new selection model relies on all characters lining up perfectly on the grid https://github.com/sourcelair/xterm.js/pull/670

Ideally line reflow https://github.com/sourcelair/xterm.js/issues/622 would be done before this so it's easier to change the contents of multiple lines.

Terminal.app:

VS Code 1.13 (notice sentences are reversed):

@mostafa69d @CherryDT a little info on the languages in question would be handy:

1. Where should the strings be flipped.for Hebrew/Arabic/Persian, do I reverse entire continuous sequences of characters in-between ascii characters?
2. How are the characters meant to interact with characters like 0-9 or punctuation?

Useful references:

• http://www.i18nguy.com/temp/rtl.html
• https://en.wikipedia.org/wiki/Arabic_script_in_Unicode
• https://en.wikipedia.org/wiki/Syriac_(Unicode_block)
• https://en.wikipedia.org/wiki/Hebrew_(Unicode_block)

Start by reading the linked Unicode and RTL references, then review the character-width change in issue 467 and the selection model in pull request 670. Check the dependency on line reflow in issue 622 and clarify the behavior around Hebrew, Arabic, Persian, digits, punctuation, and ASCII boundaries. Done means RTL text renders in the correct direction while characters remain aligned on the terminal grid.