[Feature Request] Incorporate `slack-advanced-exporter` and `mmetl`

[icelander]

The current process to import data from Slack involves two separate binaries to first download file attachments and emails and then convert to the Mattermost Bulk Export format: slack-advanced-exporter and mmetl.

It would be a much better user experience if mmctl could handle importing from Slack directly by incorporating these two binaries into a single command, e.g. mmctl import process slack <slack import file>

[isacikgoz]

Hey @icelander sounds like a good improvement but since the import/export format can change overtime IMO it would be better to keep thing separated.