Add maven plugin plugin

Added `help` goal to show usage. Examples are here:

```zsh
% mvn jp.co.worksap.jax_rs:js-generator-for-jax-rs:1.0.1-SNAPSHOT:help
[INFO] --- js-generator-for-jax-rs:1.0.1-SNAPSHOT:help (default-cli) @ js-generator-for-jax-rs ---
[INFO] a Maven plugin to generate JavaScript for JAX-RS 1.0.1-SNAPSHOT


This plugin has 2 goals:

js-generator-for-jax-rs:generate
  Generate JavaScript code which connect to JAX-RS Server. It uses Java
  controller to generate JavaScript code, and put them into specified directory.

  See also: https://github.com/WorksApplications/js-generator-for-jax-rs

js-generator-for-jax-rs:help
  Display help information on js-generator-for-jax-rs.
  Call mvn js-generator-for-jax-rs:help -Ddetail=true -Dgoal=<goal-name> to
  display parameter details.
```

```zsh
 % mvn jp.co.worksap.jax_rs:js-generator-for-jax-rs:1.0.1-SNAPSHOT:help -Ddetail -Dgoal=generate
[INFO] --- js-generator-for-jax-rs:1.0.1-SNAPSHOT:help (default-cli) @ js-generator-for-jax-rs ---
[INFO] a Maven plugin to generate JavaScript for JAX-RS 1.0.1-SNAPSHOT


js-generator-for-jax-rs:generate
  Generate JavaScript code which connect to JAX-RS Server. It uses Java
  controller to generate JavaScript code, and put them into specified directory.

  See also: https://github.com/WorksApplications/js-generator-for-jax-rs

  Available parameters:

    dataNameToGetContextPath
      Name of data attribute to get context path. If your page has a meta tag
      like below, this parameter should be 'context-path'.

      <meta name='app-data' data-context-path='context-path'>
      Required: Yes

    metaTagName
      Name of meta tag to get context path. If your page has a meta tag like
      below, this parameter should be 'application-data'.

      <meta name='application-data' data-context-path='context-path'>
      Required: Yes

    outputDirectory
      Directory where generated scripts will be stored.
      Required: Yes

    packageName
      Package which target controllers belong.
      Required: Yes

    project

      Required: Yes
```