Add the ability to add/remove params

Hi, I liked your extension more than [URL Parameters](https://chrome.google.com/webstore/detail/url-parameters/mbnigpeabbgkmbcbhkkbnlidcobbapff) (yours has a better look and feel), but one can't add or remove parameters. With my addition this is now possible. 

I think it's a good idea to allow one to edit the URL's path the same way the params can be edited. Many people use REST and/or SEO URLs, which move the would-be query string to the path.

A couple other suggestions: 

1. Move the `Use window URL` button below the textarea, this will make the URL easier to read. Now it's a little squished. 
2. Always process the current tab's URL when the extension is shown. This saves the user from always having to click `Process URL`. If they want to paste and process their own URL then they should paste it and click `Process URL`, you can't avoid this but you *can* avoid the former.