Add option to automatically build dependency charts before schema generation

[sunnstix]

As per the docs,

If you'd like to use helm-schema on your chart dependencies as well, you have to build and unpack them before. You'll avoid the "missing dependency" error message.

For those of us who are not storing our dependent charts as part of our repo and would prefer not to have to worry about the process of rebuilding and unpacking the dependency tarballs I think it would be helpful to add a flag i.e. --build-dependencies that would handle the process of rebuilding dependency charts and unpacking them for schema generation.

Seems like a fairly simple feature, but I will investigate the semantics if this seems in scope for this tool.