macOS support improvements

[Stealthii]

This PR restores macOS compatibility, and fixes a few remaining issues with current widgets.

Major changes

• [X] Support macOS in netspeed widget

Minor changes

• [X] Use calculated data from nowplaying-cli, fixes Unicode support
• [X] Use GNU date to format duration/position in music-tmux-statusbar

Bug & security fixes

• [X] Appropriately handle now-playing durations above 1 hour
• [X] Use GNU coreutils on macOS if available

Additional information

A dependency on GNU coreutils/bc, and Bash 4+ is implied with this update. The bash requirement only affects macOS at present, however Homebrew instructions are being provided for dependencies and use of this package manager is assumed to provide tmux and other utilities that are already used by this theme.