Configurable parsing and formatting API

[ariebovenberg]

One important feature is the ability to parse datetimes in various formats.

Some initial thoughts:

• The main API would be .parse(format, s) using
• potential convenience wrappers could exist: .parse_strptime(format, s) and .parse_dateutil(s) (if dateutil is installed)

[ariebovenberg]

Note: A customized formatting API would of course be the inverse of this, and probably developed at the same time.