Add macOptionSelectionMode option

[chabou]

Two possible value:

• 'vertical' (Default): enable column selection when Option key is hold
• 'force': force selection regardless of whether the terminal is in mouse events mode

[juancampa]

It would be cool if this was available on all platforms and not just mac, but I'm seeing that xterm.js implemented it as a mac-only thing. Oh well.

[Tyriar]

@juancampa I think shift or something forces on Linux, it's a mac only option as it's a conflict only on mac.

[juancampa]

@Tyriar, thanks for clarifying! 🙌