Allow syslog parser to either match multiline or provide custom regexes

Check [CONTRIBUTING guideline](https://github.com/fluent/fluentd/blob/master/CONTRIBUTING.md) first and here is the list to help us investigate the problem.

**Is your feature request related to a problem? Please describe.**

Although technically it is not to spec, sometimes syslog input manages to sneak in some newlines in syslog messages. That is not a problem, it is actually very nice, but it does tend to confuse the syslog parser, which cannot parse such messages.

**Describe the solution you'd like**

Solution to immediate issue is pretty simple, simply adding multiline flag (/m) to the regex fixes it, but I could not find an easy way to replace the regex in the standard syslog plugin.

It would be nice if one or more of these can be done:

1 - Multiline flag added to the hardcoded regexes for syslog parser. I don't think this would break anything.
2 - A configuration parameter is added to parser config that optionally adds the multiline flag, in case there is still fear that 1 will break something
3 - Configuration parameters are added that would allow to optionally replace hardcoded regexes

**Describe alternatives you've considered**

An obvious alternative is to not use syslog and use regex parser, They are not that far apart and it should be easy, but syslog offers one thing that is handy - detection of which rfc format message is in before parsing. That too can be implemented without syslog plugin, but it will lead to an unnecessarily convoluted configuration

Thank you.