Added this.rectMode support

[sahej-dev]

I have added support for all P5 rectModes. All the following are valid:

myButton = new Clickable();
myButton.rectMode = CORNER; // Default
myButton.rectMode = CORNERS;
myButton.rectMode = CENTER;
myButton.rectMode = RADIUS;

• Click area, button rect, image, and text scaling should be working, as expected with all the rectModes.
• Changing rectMode by doing (for e.g. rectMode(CENTER) should no longer affect clickables. myButton.rectMode = CENTER must be used.
• I have also update the minified version with the latest version of the library.

I have tried not to change up the existing structure of the library too much. Please let me know if I should make any further improvements/changes to this PR.