Add flag for config path

[sashayakovtseva]

Hi,
One thing which can make prototool even better is custom config path.
In large projects there may be many configs for different tools and it would be extremely convenient to keep them all in a dedicated directory (e.g. configs) rather than cluttering repository's root.

Imagine the following project structure:

repo/
  api/
    api.proto
  cmd/
  configs/
    prototool.yaml
  internal/
  pkg/
  README.md

Working directory is repo root.
The config path flag would be passed like: prototool generate --config configs/prototool.yaml
Prototool should work the same as if the config was in the working directory (i.e. repo root).

Please let me know what you think about this feature request.
P.S. There is a PR open already #519, but it seem to be left without attention.