Add module-info.java

[zimmi]

Hi!

I'd like to use fastexcel in a modularized application, so it would be fantastic if this project could add a module-info.java file.
Since it already requires Java 9, this should be as simple as putting a file like this into the package root:

module com.github.rzymek.opczip {
    exports com.github.rzymek.opczip;
    exports com.github.rzymek.opczip.reader;
    exports com.github.rzymek.opczip.reader.ordered;
    exports com.github.rzymek.opczip.reader.skipping;
}

Open questions:

1. Should every package be exported?
2. Are you open for a PR doing this? :)

I see that fastexcel itself has a baseline of Java 8, but there is a straightforward way to add a module-info.java there as well: https://maven.apache.org/plugins/maven-compiler-plugin/examples/module-info.html

[rzymek]

Sure! I'm totally open for PRs!
I think every package should be exported.

We can have module-info.java and still have Java 8 compatibility, so that's not a problem.