Add pre-release flag to GitHub releases

## Description
Currently, chart-releaser doesn't provide a way to mark GitHub releases as pre-releases. This feature would be useful for charts that are still in development or testing phase and shouldn't be considered production-ready.

## Proposed Solution
Add a new `--pre-release` flag to the `upload` command that allows users to mark GitHub releases as pre-releases.

## Implementation Details
The implementation would require:
1. Adding a new boolean flag `--pre-release` to the `upload` command
2. Adding a new field `PreRelease` to the `Options` struct in the config package
3. Adding a new field `PreRelease` to the `Release` struct in the github package
4. Passing the pre-release flag value to the GitHub API when creating releases
5. Updating tests to cover the new functionality
6. Updating documentation in README.md

## Benefits
- Allows users to clearly mark pre-release versions of charts
- Provides better integration with GitHub's release features
- Helps users distinguish between stable and unstable chart releases

## Acceptance Criteria
- The `--pre-release` flag is available in the `upload` command
- When the flag is set, GitHub releases are marked as pre-releases
- Documentation is updated to reflect the new feature
- Tests are added to verify the functionality