Support for multiline entries

While migrating some of my cronjobs to supercronic, I noticed that multiple lines joined together with the `\` character work in cron, but are not supported in supercronic.

For example:

```
*/10 * * * * echo 'a' && \
echo 'b' && \
echo 'c'
```

Is a valid crontab entry for most classic crons, but is rejected by supercronics parser.

Start by tracing supercronic's crontab parser, since the issue identifies parsing as the failing entry point. Compare its handling of continued lines with the shown cron example. Done means a valid multiline entry using backslashes is accepted rather than rejected.

Written by the indexing model from the issue text.