Lint plugin files.

With gulp-jshint.
Add a sbt task to run JavaScript console.

The task launches JavaScriptShellConsole in beyond.tool.
Add task to test plugin JavaScript APIs.

Implement a sbt task to test plugin JavaScript APIs. The test framework
is written in JavaScript and placed in `plugins/test/lib`. The task
finds test cases in the `plugins/test` directory. By bdd.js, the test
cases are parsed as a task tree and then launched. The result is
reported by TestReporter written in Scala.

An example test case, `example.js`, is added in `plugins/test` to show
how a test case should be implemented.
Fix sep and delimiter properties in ScriptablePath.

JavaScript function's static properties can't be defined with @JSGetter.
We should define the properties in the `finishInit` function.
Add test case for ScriptablePath API.

It can be tested with `activator plugin-test`.