Log Reader: Full nested folder structure shouldn't be required to get date

[Antar1011]

For testing / dev purposes or just in general, a JsonFileLogReader should be able to pull date information from somewhere besides the folder where the log is stored.

Proposed solution: give JsonFileLogReader a date field that it can fall back to if parsing the folder structure doesn't make sense.

[Antar1011]

Beginning work on more-flexible-date-parsing branch

[Antar1011]

Also submitted a PR to PS: smogon/pokemon-showdown#2732

Ofc, with the formats field there's a circular problem here if the strategy is separate readers for separate formats--how do you know which reader should read the log until you read the log?

[Antar1011]

Just need to add some tests for date-parsing and exception-raising.