Add --from and --to time range flags

[Copilot]

Users had no way to bound a dump to a specific time window, requiring full channel/thread exports even when only a slice of history was needed.

Changes

• main.go: Registers --from and --to string flags; adds parseTime helper accepting RFC3339 or YYYY-MM-DD; replaces sd.DumpAll() with sd.Dump(ctx, link, oldest, latest) — zero time.Time{} when a flag is omitted preserves existing unbounded behaviour
• main_test.go: TestParseTime covers empty input, RFC3339, date-only, and invalid formats
• README.md / cobra Long/Example: Documented new flags

Usage

# Date-only bounds
gh slackdump --from 2024-01-01 --to 2024-01-31 https://myworkspace.slack.com/archives/C09036MGFJ4

# Precise RFC3339 bounds
gh slackdump --from 2024-01-15T09:00:00Z --to 2024-01-15T17:00:00Z https://myworkspace.slack.com/archives/C09036MGFJ4

# Either flag is optional; omitting both dumps everything (unchanged behaviour)

Original prompt

Add support for specifying a time range for the Slack dump in the repository wham/gh-slackdump. This should allow the user to define a start and end time for the dump process, enabling them to extract messages only within the specified range.

This pull request was created from Copilot chat.

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