Update submit command to filter implicit file list against allowed extensions

This depends on https://github.com/exercism/cli/issues/609

As part of https://github.com/exercism/meta/issues/107 we will need to update `cmd/submit.go` to filter against the accepted list of file extensions. This is in addition to the existing filter to _exclude_ based on the blocklist.

Caveats:

- We will _only_ filter if the user is submitting a directory. If they're submitting a list of files, we will trust them to know what they are doing.
- If the list of extensions is empty, we will only use the blocklist.