improve ISO8601 parsing

[jftuga]

Purpose

I have added the capability for the parsetime library to parse inputs in the following format:

2006-01-02 15:04:05 MST

Changes

This was done by adding the following code to the parseISO8601 function:

if len(group) == 10 && group[9] != "" {
	loc, err = toLocation(group[9])
	if err != nil {
		return t, priority, err
	}
}

The zone regular expression also needed to be updated. Are you in agreement with this change? All tests still pass (see below).

zone = `([a-zA-Z0-9+-]{3,6})?`  // Added parentheses to create a capturing group

I also fixed a misspelled word: February.

In order to add this feature, I needed to understand the code base. To that end, I asked claude.ai to generate comments for most of the functions. I then read over these comments and they seem accurate. I can remove these from the PR if you don't want them in there or just remove the Example usage sections of the comments.

Testing

I added two tests in parsetime_test.go to verify the new format. All tests passed when running go test. Can you please verify that these are good tests?