Open ended range time of day subset

[joshuaulrich]

@ckatsulis asked via email if time of day subsets should support an open-ended range like daily (or higher) subsets do. For example x["T23:00/"] would subset an xts object from [11PM, 12AM).

This would be a convenient way to specify something like data["T23:00:00/T23:59:59.999"] without being as verbose.