Assertion file template for IntelliJ.

Provide file template which helps writing custom assertions. More lightweight approach than [cli / maven generators](https://github.com/joel-costigliola/assertj-assertions-generator/).

Perhaps once the [Eclipse plugin](https://github.com/joel-costigliola/assertj-eclipse-plugin) gets stable it could act as a reference for an IntelliJ plugin.

Merge would require updating links to Eclipse code formatter in [assertj-examples](https://github.com/joel-costigliola/assertj-examples) (README.md and CONTRIBUTING.md).