Allow per-command configuration of custom secrets dir (#247)

Typical use case is when you want different people to have access
to some files and other people access to other different files.

src/_utils/_git_secret_tools.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 
 # Folders:
-_SECRETS_DIR=".gitsecret"
+_SECRETS_DIR=${SECRETS_DIR:-".gitsecret"}
 _SECRETS_DIR_KEYS="${_SECRETS_DIR}/keys"
 _SECRETS_DIR_PATHS="${_SECRETS_DIR}/paths"