Switch to commands instead of flags

[rusq]

There's a repelling amount of command line flags right now, it would make sense to implement the major modes, such as:

• export
• list
• dump (standard)
• etc.

As separate commands.

For example, current invocation for export is:

slackdump -export foo.zip -export-type mattermost -download

would be

slackdump export -type mattermost -download foo.zip 

Alternatively, as suggested in #121 could you the https://github.com/spf13/cobra library