feat: bgcolorEnd option for color progression as progress increases

[franverona]

Overview

Add a bgcolorEnd option that interpolates the overlay's background color from bgcolor toward bgcolorEnd as progress increases from 0 to 100.

Proposed API

// Vanilla
Loadgo.init(el, {
  bgcolor: '#FF0000',    // red at 0%
  bgcolorEnd: '#00FF00', // green at 100%
})

// jQuery
$('#logo').loadgo({
  bgcolor: '#FF0000',
  bgcolorEnd: '#00FF00',
})

Behaviour

• When bgcolorEnd is set, each setprogress call linearly interpolates the overlay's backgroundColor between bgcolor (at 0%) and bgcolorEnd (at 100%).
• When bgcolorEnd is not set (default null), behaviour is unchanged — overlay color stays fixed at bgcolor.
• Interpolation operates in RGB space on each channel independently.
• Both hex (#RRGGBB) and rgb(r, g, b) formats should be accepted for both bgcolor and bgcolorEnd.
• Not applicable in filter mode (no overlay exists); option is ignored in that case.

Motivation

Enables at-a-glance progress feedback through color without requiring custom CSS or an onProgress callback:

• Red → green for success indicators
• White → brand color for reveal effects

Notes

• Both loadgo.js (jQuery) and loadgo-vanilla.js should be updated.
• Composable with animated / animationDuration — the CSS transition on the overlay already covers background-color if added to the transition property list.