Replace System.err & System.out with proper logger API

[KengoTODA]

Current implementation uses System.out and System.err to output debug information, but it should be avoided in production code. Because using stdout and/or stderr directly makes system maintenance difficult, it doesn't support filtering base on log level nor class. It is also hard to apply log rotation and other log management mechanism.

I cannot judge which logger API we should use, probably it is one of followings:

1. SLF4J API, which is common in Java ecosystem
2. JUL (java.util.logging) API, which is standard in Java but little bit hard to maintain
3. Log4J 2 which is used by Elasticsearch core

[KengoTODA]

It is OK to use System.err and System.out in main class such as SudachiCommandLine.java and PrintDictionaryHeader.java. But at least DictionaryBuilder.java should use logger interface.

It seems that SLF4J is not so active recently, then I recommend to use Log4J 2.

[kazuma-t]

I expect to use DictionaryBuilder.java and UserDictionaryBuilder.java on the command line. Should I use a logger for them?

[KengoTODA]

Then not high priority I think.

If I was you, I will introduce logger system eventually. It makes product easy to introduce --debug, --verbose or other flags with keeping performance and readability.