Query payload keys only once
Define `extract_format` during "compile time"
Modify data directly avoiding usage of Hash#merge!
Remove needless data filter in key value formatter

We don't need to filter data because we skip them anyway
if the key does not exist.
Avoid `reduce`. Just use `map`
An error is always a String never a Float
Prefer string index + substring over split
Remove unused variable event
Update changelog
Merge pull request #9 from splattael/minor-refactoring

Performance optimizations
Merge branch 'master' into timestamp

Conflicts:
	lib/lograge/formatters/key_value.rb
	lib/lograge/log_subscriber.rb