Add highlight line feature to CodeMirror

highlight feature:
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When one line is clicked, (the line will have the cursor), this line will add highlight style (ie., add class "highlight-line" to the line).

addLineClass and removeLineClass are used in this plugin. Note removeLineClass function will not run on every line. It only runs once. This is very fast than other ways.

The demo page: /demo/highlightline.html

![image](https://f.cloud.github.com/assets/168751/637349/d7f0962a-d2a1-11e2-9c67-66c6c94b4c2c.png)