DOC: more info on developing filters

Well more documentation was needed so this is a bit better.

I've put in a plug for debuggex.com as its really useful for filter development.

@stoarca in #284 you mentioned "I'm particularly fond of open-source projects using it, so I will try to make it as accessible as possible".  I'll probably buy myself an individual subscription which will probably be 99% for fail2ban development however I'm wondering what arrangement is suitable for contributors.

Can we do a limited time development for X open source project? So in our documentation we do some kind of url http://debuggex.com/?trial=fail2ban where our uses can play with it. You can I assume do some captures of data to see if its really being used for fail2ban log message regexs development?

I suspect an individual or companies may wish to do a donation/tip for the service so can we provide a url for that? Is there another way that we can promote your service?

I really like your recent redesign putting the regex and test data beside each other and the addition of (?&.ipv4).

I want your business model to succeed and I'm also keen continuing using debuggex being available for potential fail2ban filter contributors at a low cost. What options do you see that are appealing to you?

Small feature request while I think of it - Can there be a url that defaults to Python regexes?