Add custom ruby version manager

[jonjonw]

It would be helpful if you could add a 'custom' option for ruby version manager that runs a specified command to activate a custom version manager.

See https://github.com/Shopify/ruby-lsp/issues/1571 in the vscode-ruby-lsp extension for an example.

This would be helpful as I use direnv to manage my ruby version and it needs to be activated to set the PATH for this plugin.