2.3.0-alpha.3

• add settings for table columns, useful to hide columns in small view-ports
• replace multi-sort function (no more eval(), yay!)
• use latest Bootstrap 4 class names
• rename Open/View button to "Download"
• add download attribute to Download button
• add display_errors setting
• add scroll-to-top when search field is focused through shortcut
• add new file-types
• rewrote JavaScript from scratch
• remove alt_load setting
• remove auto_highlight setting
• replace all gmdate() instances with date()
• add aria-hidden=true to file-icons
• bump Bootstrap to v4.0.0-alpha.3
• bump jQuery to v3.1.0
• bump highlight.js to v9.5.0
• various Gulp task fixes