Compute start and end offsets from timestamps

[faillefer]

Description

This PR is based on PR #126 by @nanic (Thanks for the work !)
Implements #121

I've added integration tests and updated the code so that '--exit' is not mandatory when using --from-timestamp
I've had to rework getOffsetBounds method to better manage possible options
Also I removed the usage of '-1' value when returning offset when there's an error, because '-1' is a meaningful value for kafka (OffsetNewest)

Type of change

• New feature (non-breaking change which adds functionality)

Documentation

• the change is mentioned in the ## [Unreleased] section of CHANGELOG.md
• the configuration yaml was changed and the example config in README.md was updated
• a usage example was added to README.md
• tests for the changes have been implemented (see: Testing your changes)

[d-rk]

Generally the PR looks good 👍
I found mainly minor issues. The only thing that I like to be added is the parsing of different timestamp formats.

[d-rk]

when reading <timestamp-with-milliseconds> it is unclear what the parameter really expects. It could be an iso timestamp (e.g. 2015-08-29T11:22:09.815Z) or epoch milliseconds.

I would like to have it as intuitive as possible. So I would suggest to change the parameter from an int64 to a string and parse different timestamp formats.
The parsing could be done with a small helper function like parseTimestamp below:

var dateFormats = []string{
	"2006-01-02T15:04:05.123Z",
	"2006-01-02T15:04:05.123",
	"2006-01-02T15:04:05Z",
	"2006-01-02T15:04:05",
	"2006-01-02T15:04",
	"2006-01-02",
}

func parseTimestamp(timestamp string) (time.Time, error) {

	if timeMs, err := strconv.ParseInt(timestamp, 10, 64); err == nil {
		return time.UnixMilli(timeMs), nil
	}

	for _, format := range dateFormats {
		if val, e := time.Parse(format, timestamp); e == nil {
			return val, nil
		}
	}
	return time.Time{}, errors.Newf("unable to parse timestamp: %s", timestamp)
}

func TestTimestamp(t *testing.T) {

	var examples = []string{
		"2014-04-26T17:24:37.123Z",
		"2014-04-26T17:24:37.123",
		"2009-08-12T22:15:09Z",
		"2017-07-19T03:21:51",
		"2013-04-01T22:43",
		"2014-04-26",
		"1384216367189",
	}

	for _, example := range examples {

		if val, err := parseTimestamp(example); err != nil {
			fmt.Printf("Error converting %s - err %v\n", example, err)
		} else {
			fmt.Printf("converted %s to: %v\n", example, val)
		}
	}
}

[d-rk]

Suggested change

	func StrArrayEqual(a, b []string) bool {
	func StringArraysEqual(a, b []string) bool {

[faillefer]

Thanks for this very complete review and for the proposed improvements 😃. I think they are all implemented in the last commits.
It would be helpful to update the readme.md with supported timestamp/date formats. But shall we use "examples" to describe them (as in Go time package) or a more standard pattern (like yyyy-MM-dd'T'HH:mm:ss.SSS)

[d-rk]

thank you for the PR @faillefer 👍