Configurable date format

[eggrobin]

Fix #2, and make the epoch configurable too.
The date format config is of the following form:

RSSTimeFormatter {
  dateFormat = <insert format string here>
}

Example format strings: dd.MM.yyyy, M/d/yyyy, dd/MM/yyyy, yyyy/MM/dd, yyyy. M. d..

[jwvanderbeck]

Has this been tested locally?

[eggrobin]

For dateFormat:

• I have checked that absent a config the default is used and works;
• I have checked that a config with dateFormat = dd.MM.yyyy (frequent in central and eastern Europe) has the intended effect.

For epoch:

• I have checked that absent a config the default is used;
• I have checked that with epoch = 1993-01-01T00:00:00 dates are suitably offset.

Note that in practice epoch should be combined with something to offset the initial positions of the celestials in-game, e.g., Principia's game_epoch setting. Principia parses dates that are either Julian dates or a fairly strict subset of ISO 8601, which is why I specifically checked the 1993-01-01T00:00:00 format for the epoch of RSSTimeFormatter, since Principia will accept that format.