Include a force option to always publish

I'm working to integrate the chart releaser action into our GitHub release pipeline and have hit an issue of conflicting assumptions. Our release pipeline is triggered off creating a new release, which creates a tag. The helm releaser action script checks for changed charts since the last tag, so you can see where the problem lies.

```
Run helm/chart-releaser-action@v1.0.0-alpha.2
env:
CR_TOKEN: ***
project
Looking up latest tag...
Discovering changed charts since 'v2.0.0a19.5'...
Nothing to do. No chart changes detected.
```

I propose something like a -force option for the case where the caller knows it wants to publish a new version of its charts.

Start by tracing the action script's changed-chart check and how it invokes the chart releaser. Add a force option for workflows that need to publish despite no detected chart changes, then verify it with a release-triggered workflow like the one described.

Written by the indexing model from the issue text.