Add support for precise datetime parsing

[m-haisham]

The parse currently only understands relative datetimes. The previous update (#5) added support for adding and subtracting duration from DateTime using + and '-' respectively. This currently lacks support for parsing precise DateTime strings such as 18:20:30.

This should add support for parsing the following strings.

• 12:00
• 12:30:00
• a hour from 00:00
• 12 minutes 5 seconds + 12:00

This issue is derived from #4.