Add --exclude-dependents boolean flag

[br0p0p]

Hi,

Thanks for creating the changed plugin!

We're using this in CI at work and we don't version everything in lock step so I'd like a way to avoid including dependencies in the list of changed workspaces.

The default behavior is unchanged but with this update I can do the following:

yarn changed list --git-range origin/master --exclude-dependents

Definitely open to naming changes on this. Let me know what you think.

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I think the root issue might actually be that in our monorepo all packages rely on the root package so any changes there count as a "dependent" change for all packages. Maybe adding a flag to ignore the root package's changes when calculating dependents is the better way to go. I see someone already brought that up in #6

Interested to hear your thoughts.