Handle command `i`, `a`, `c` in multiple -e

GNU sed can handle a single command `i`, `a` or `c` specified in multiple `-e` calls, sedparse cannot.

```console
$ seq 5 | sed -e '3i\' -e foo
1
2
foo
3
4
5
$ sedparse -e '3i\' -e foo
sedparse: -e expression #2, char 1: unknown command: `f'
$
```