Keyboard arrows to change the value inside parentheses

Keyboard up/down to increase/decrease value that is inside parentheses containing browser caret.

By default, delta is 1 for integers and 0.1 for floats.
Use `shift` for x10, `ctrl` for x0.1.

Tested in Firefox and Chrome.

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Preview at http://jakub-g.github.io/csstransformer/

Actually I should have opened this PR on `gh-pages` branch. BTW you can get rid of master and set `gh-pages` to be default branch in https://github.com/captainbrosset/csstransformer/settings