Allow for override of gui external runner

[Attamusc]

I've been using vim-rspec for a while and it's awesome, but the recent update to allow for both g:rspec_runner and g:rspec_command breaks my current workflow that I think others might also have: MacVim and tmux. Currently, all commands when run in gui vim are executed using the configured runner. This means that, even with a custom command, gui vim will not be able to execute anything inside of vim to externally run the command.

I added a global variable that allows the user to explicitly state if they do or do not want to use a runner script. This variable is set to true by default so that the current behavior without setting either rspec_command or rspec_runner remains the same.

While I don't think this is necessarily the best way to do this, it is "a way" and it requires minimal change. If there is a better way to do this, please let me know; if not, I'd be happy to also update the README with references to this variable.

[gylaz]

What is your use case that you don't want to use an rspec_runner with GUI vim?

[Attamusc]

I currently have a set up like in this article, where vim-rspec is set up to pipe the command to tmux using tslime.vim

[gylaz]

@Attamusc I've opened #94 which should address this. Do you mind trying that branch to see if it solves your problem?