add tags for rspec via system properties

jruby-gradle-rspec-plugin/src/main/groovy/com/github/jrubygradle/rspec/RSpec.groovy

@@ -61,10 +61,17 @@ class RSpec extends DefaultTask {
         if (pattern != null) {
             args += ['--pattern', pattern]
         }
+
+        String tags = System.getProperty("${name}.tags")
+        if (tags != null) {
+            tags.split(/\s+/).each { args += ['--tags', it] }
+        }
+
         String file = System.getProperty("${name}.file")
         if (file != null) {
             args += [file]
         }
+
         jruby.exec(args)
     }
 }

jruby-gradle-rspec-plugin/src/test/groovy/com/github/jrubygradle/rspec/JRubyRSpecPluginSpec.groovy

@@ -286,4 +286,17 @@ class JRubyRSpecPluginSpec extends Specification {
         then:
             thrown(ExecException)
     }
+
+    def "Run rspec with unknown tag"() {
+        given:
+            Files.createSymbolicLink(specDir.toPath(), new File('src/test/resources/simple/spec').getAbsoluteFile().toPath())
+            project.evaluate()
+            System.setProperty('rspec.tags', 'me-and-the-corner:today')
+            Task task = project.tasks.getByName('rspec')
+            String output = captureStdout {
+                task.run()
+            }
+        expect:
+            output.contains( '4 examples, 0 failures' )
+    }
 }