New input to specify an index directory

[isaaguilar]

I'm working on a repo where the index.yaml is not stored in a conventional directory. This adds a new input to specify an index directory.

[unguiculus]

Thanks for the PR. I'm hesitant to expose options directly. #50 adds an option to specify a config file. chart-releaser will already discover a cr.yaml in the current directory. You could also use an environment variable CR_INDEX_PATH to achieve what you want.

[isaaguilar]

@unguiculus Cool thanks for the update. I'll keep my eye out for #50 to drop so I won't need my little tweak anymore.