Add parsing for timestamp syntax

[whacked]

As far as I can tell, there is no timestamp syntax handling in the current version.

A variety are described in the 8.1 Timestamps, deadlines, and scheduling in the manual (as of 2016-02-24)

The document page does not show other formats like ISO 8601 and similar, though native org parses it as "clock time". Examples:

• [2016-02-21 10:07:07-0500]
• [2016-02-24]
• [2016-02-24 Wed]
• [2001-07-08 01:23:45]

Since the datetime cannot span more than one line, I am guessing this needs to be something like a Syntax.define("date"...) entry in https://github.com/mooz/org-js/blob/master/lib/org/lexer.js ?

The regex for schedule/deadline parsing in daiganio's org-mode-parser.js is here:
https://github.com/daitangio/org-mode-parser/blob/master/lib/org-mode-parser.js#L216-L217

The regex for "clock time" parsing for the same library is in https://github.com/daitangio/org-mode-parser/blob/master/lib/org-mode-parser.js#L226 (however, for org export, clock time does not need to be preceded with the "CLOCK:" string and can occur anywhere)

I am happy to give a try of adding a lexer / exporter entry for a simple version of this, but would like to know what you think.

In particular, if you want larger coverage, Timestamp with repeater interval and Diary-style sexp entries require additional processing after the regex match, so I don't know if my Syntax.define guess is correct.

Thanks!

[whacked]

master...whacked:2016-06_exploration has a rough cut of adding timestamp parsing