• keep minimal Go version at 1.13 (and not 1.16 which is more convenient for cmd)
• cmd: fix regression for stdin/stdout, fixes #392
• HTML: update error line and column for errors from other minifiers
• CSS: KeepCSS2 (for IE11) doesn't use initial for background-color, fixes #394
• CSS: keep integers for z-index even in functions such as calc, fixes #395
• CSS: fix minify to initial/auto/none for flex, fixes #398