filebeat versions support

[urso]

filebeat 1.x has been using regexp.CompilePosix, 5.x switched to regexp.Compile and in 5.3 it's using is using match.Matcher optimizing some common patterns.

As all versions are commonly used, would be nice if the filebeat-multiline-tester would support a setting for choosing the actual pattern-backend.

[hartfordfive]

@urso Thank you for pointing that out. I will take a look at making the necessary modifications for this as soon as possible.

[hartfordfive]

@urso Sorry for have taken so long to get around to this. I started the modifications for this and I have a PR open if you want to check it out:
#3

I'll try to finish this up over the next few days and create a release of it. If you see any issues, details and/or clarifications I'm missing, please let me know.