skip hidden directories (start with .) picked up by -f

there is probably a common case for ignoring hidden directories by default.

common case within carvel tools might be: imgpkg pull results in a directory with .imgpkg directory. if bundle's files arent wrapped within a directory (eg config), then running `ytt -f .` would include `.imgpkg/*.yml` which isnt really desired within templates.

another example: currently ytt -f . that includes .git directory actually makes .git files available via data.read and friends. it just happens that there is no .yml files within .git by default.

what other examples of hidden directories ytt may encounter? it might be worth adding a way to control this behaviour?

Start with the `ytt -f .` CLI behavior and trace how directory inputs are discovered and passed to `data.read`. Review the issue's examples involving `.imgpkg` and `.git`, then define the default handling and any control mechanism before implementing; done means hidden directories no longer enter templates unexpectedly and the chosen behavior is covered by tests and documentation.

Written by the indexing model from the issue text.