Set angle to max at .5 to prevent graphical errors

Turns out if you set angle to anything greater than .5, the line will start doubling back on itself and weird errors occur. After playing around with different values for a bit, I can't find a use case where you would need the angle to be greater than .5.

Here, I've modified `BaseGauge.setOptions()` with a conditional that ensures `gauge.options.angle` will never be greater than .5.

I also updated the demo page to reflect that change and set a max value for the angle slider.