fix parallax bugs and add build script for v1.5.0

• Fix background offset not updating on scroll (#77)
• Fix element jump on window resize (#57)
• Fix factor:0 at breakpoint not disabling parallax (#76)
• Fix foreground element jump on initial scroll (#61)
• Fix parallax triggering before element enters viewport (#79)
• Fix negative/inverse factor not working (#60)
• Add npm build script using terser to generate minified dist
• Bump version to 1.5.0