Allow additional configurations to manually extend the crontab execution command

Right now, the configuration only allows to adjust the following two parameters:

| Config item | Description | Default value |
| ------------------ | ------------------------------ | ------------------ |
| CRONTAB_EXECUTABLE | The absolute path of `crontab` | `/usr/bin/crontab` |
| CRONTAB_LOCK_JOBS | Whether lock jobs when running | `False` |

However, it would be great if there are two additional configuration variables that allow to add a prefix and a suffix to the command. For example, this could look like the following (taken from the [django-crontab](https://github.com/kraiz/django-crontab) project):

**CRONTAB_COMMAND_PREFIX**

* something you want to do or declare, before each job gets executed. A good place for environment variables.
* default: '' (empty string)
* example: 'STAGE=production'

**CRONTAB_COMMAND_SUFFIX**

* something you want to do after each job was executed.
* default: '' (empty string)
* example: '2>&1'

That would allow me to redirect the output stream with the suffix and set important environment variables with the prefix configuration.