Switched back to 2 spaces indentation
Renamed 'aa' option to 'timecode'

This is to conform to the style adopted in the semanticdata extension,
develloped in parallel.

the 'aa' option could then be a switch to enable them all.
The timecode regex is more versatile
Little aesthetic change in the timecode regex
Changed the timecode option so one can pass a function to format the
timecodes

For instance, if you have a toSeconds String prototype method:

```javascript
var src = ''
+ '00:00:00 --> 00:00:10\n'
+ '\n'
+ 'test'
+ '\n'
+ '00:00:05 --> 00:00:15\n'
+ '\n'
+ 'test'
+ '\n'
;
marked.setOptions({
    timecode: function(tc) { return tc.toSeconds() }
});
var result = marked(src);
```

If the value for the timecode option isn't a function, it will still
test if it evaluates to True, so you can still use:

```javascript
marked.setOptions({
    timecode: true
});
```