Make grok pattern PROG a bit more flexible

This small patch makes logstash correctly interpret e.g. ansible's "ansible-<stdin>" progname. ATM it failes with a grok parse error.

Generally any characters are allowed except [ and : (colon needed because sometimes a [pid] isn't used).

Tests pass:
```
$ make test
[...]
Finished in 43.47 seconds
636 examples, 0 failures
```