Filter with closures

[cholcombe973]

Thanks for building ftlog! I've customized it a little bit with this PR. It allows 2 types of closures (filters) to be added. The first type will drop logs before they're ever added into the pipeline. Could be useful for really spammy log entries. The second type allows setting a closure to select the Appender to use. This allows the user to use any criteria they want to redirect logs. The background for this change is I have a desire to redirect log entries based on a key/value pair that's in the log message. So instead of just filtering on the level I want to filter on the message itself.
I also added in some cargo clippy fixes and some little cleanup things.

[nonconvextech]

Could you please use cargo fmt to format the code?

[nonconvextech]

The idea feels great. I will test the performance impact before merging.

[rrichardson]

Since this breaks API compatibility, I might recommend adding a method in addition to filter instead of changing the filter method. Maybe filter_with.

[cholcombe973]

I merged in the filter_with functionality from Rick. I think this should be good to go.