Create platform-dependent artifact

[erip]

Consumers of sentencepiece-jni need to depend on an artifact that is compatible with their desired platform. This PR seeks to add a discriminating classifier to the build which allows a user to specify which platform they're targetting a la os-maven-plugin. This has implications on current users as they will need to explicitly add the platform they were targeting before a cross-platform build was enabled (i.e., linux-x86_64).

[erip]

See here for an example of the new artifact name with discriminating information for consumers.

[erip]

I think this is OK now @levyfan. Do you have any thoughts? The way to add this project as dependency now is, for example:

  <dependencies>
    <dependency>
      <groupId>com.github.levyfan</groupId>
      <artifactId>sentencepiece</artifactId>
      <version>0.0.1-SNAPSHOT</version>
      <classifier>osx-x86_64</classifier>
    </dependency>
  </dependencies>

which can be automated with the os-maven-plugin transparently assuming the plugin is included in the POM:

  <dependencies>
    <dependency>
      <groupId>com.github.levyfan</groupId>
      <artifactId>sentencepiece</artifactId>
      <version>0.0.1-SNAPSHOT</version>
      <classifier>${os.detected.classifier}</classifier>
    </dependency>
  </dependencies>

[levyfan]

I suppose we could further merge pom.xml and pom-windows.xml together by maven profile https://stackoverflow.com/questions/38465711/choose-maven-profile-from-os-family