Add read and write intervals

[controversial]

This would improve performance by only reading from or writing to a file if a certain time interval had passed since the last write.

A File would store information about the last time it was updated. Then, when accessed, the I/O operations would only be performed if intervial milliseconds had passed since the last update. Intervals like 100 would mean the file was refreshed up to 10 times a second. This is a small loss in accuracy, but a huge performance boost in cases like list comprehensions.

When implemented, this will probably have a default value of something like 100. However, users will be able to specify their own intervals, including 0 for completely live behavior.