Extend wildcard syntax

The current MQTT v3.1.1 wildcard syntax in a topic filter only allows the + wildcard as the only item in a level. We have frequently had customers try to use this wildcard after some characters in a level. The only way around this is to change the topic structure which is quite difficult for established applications.

My proposal is to slightly relax the restriction in the wildcard syntax to say that the plus must be the last character at a level, but does not need to be the first.

Thus we would allow something like:
my/x+/topic