add Stryker Mutator (#70)

* add @stryker-mutator/core to devDependencies
* do `npx stryker init` to configure Stryker
* add stryker task to package scripts
* update .gitignore
* add missing newline to end of stryker.conf.json

Co-authored-by: Christopher J. Brody <chris.brody+brodybits@gmail.com>
Co-authored-by: Christian Bewernitz <coder@karfau.de>

.gitignore

@@ -1 +1,5 @@
+# stryker temp files
+/.stryker-tmp
+
 /node_modules
+/reports