Using global cli option is not documented

[cubercsl]

As it is mentioned in https://github.com/wakatime/vscode-wakatime/issues/177 and https://github.com/wakatime/vscode-wakatime/pull/178, we can use some config to use global wakatime-cli provided by package manager or something else.

In this plugin, it is now use global, but not documented in https://github.com/wakatime/wakatime-cli/blob/v1.55.1/USAGE.md

Maybe we should try to find/use global by default like other plugins (such as vim-wakatime), and then fallback to automatically download.

[alanhamlett]

Sounds good, I'll start on a PR.

[alanhamlett]

Implemented in v20.0.0 released just now.