def before(*commands, &block)
      context = (@_context[:before] ||= [])
      block_given? ? run_context(context, &block) : context.concat(commands)
    end
def before(*commands, &block)
      context = (@_context[:before] ||= [])
      block_given? ? run_context(context, &block) : context.concat(commands)
    end
Run commands prior to each tab context.

 @param [Array<String>] commands
   Commands to be executed.
 @param [Proc] block
   Proc of commands to run

 @example
   # Executes `whoami` before tab with `ls` and `gitx`
   window do
     before { run 'whoami' }
     tab 'ls'
     tab 'gitx'
   end

 @api public