Implement an update_dependencies setting

[ErinCall]

In plan.go's upgrade, lint, and uninstall functions, if config.UpdateDependencies is true, there should be an additional Step that calls helm dependency update $chart. It needs to happen before the main command, but I don't think it matters whether it happens before or after InitKube.

The UpdateDependencies struct will look like Lint, though a little simpler:

• It only needs Chart and cmd fields.
• Its Prepare() method should require a nonempty Chart.
• It doesn't need to pass any global flags other than --debug.
• It doesn't need to pass any command-specific flags at all.

[ErinCall]

The draft documentation I'm writing for #8 mentions this setting but calls it out as "not yet implemented." Make sure that's updated as part of the work for this change.