Preserve the position of the caret while editing a textarea or input[type=text]

Hi, 

I came across an undesirable behavior when working with `textarea`s. The problem is that whenever a character is typed in, the caret always moves to the end of the field.

This patch fixes the problem by modifying the *webfui.plugin.form-values* plugin. The code is a bit ugly due to the restriction of plugin facility. Let me know If you know a better way to do it and I'll fix it right away.

I created an example project so that you can see the fix in action. To do this, please checkout my branch https://github.com/ympbyc/webfui/tree/textareaplugin , run the server, and navigate your browser to `localhost:8080/textarea`.

Cheers!

Minori Yamashita