Support for bootstrap/hook scripts

When developers use `graylog-project bootstrap` to clone all repositories the next step is to run `yarn install` and an initial `mvn compile`. These steps are required to run the web server and graylog server development processes from the IDE.

To avoid hardcoding these commands in the CLI, the CLI should check for hook scripts in a default location. (e.g. `hooks/`) A good default might be to check for a script that is named after the CLI command that is executed.

Examples:

- `graylog-project bootstrap`: Check and execute `hooks/bootstrap.sh` if it exists
- `graylog-project checkout`: Check and execute `hooks/checkout.sh` if it exists

In addition to that, the CLI could also check for `_custom.sh` files where developers can add some custom commands. (these files should be in `.gitignore`)