Change sort_index() to build_index() each time on execution of vee.

* Should be sh compliant
* date and time variables (incl. $PUBLISHED) may be redundant
* Could be merged with ```default_update_index()```
* ```list_newest_first()``` parses .vee, too
* ```oldest_first()``` uses output of ```ls```, which is [not recommended](http://mywiki.wooledge.org/ParsingLs)