Override for s:app_prepare_rails_command

[cvincent]

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It would be useful to have a setting to override the logic in s:app_prepare_rails_command to allow for a variety of custom setups. For example, one might want to run commands through vagrant ssh, or (in my case) echo commands into a FIFO to run them asynchronously. It might look similar to g:rspec_command from vim-rspec.

[tpope]

s:app_prepare_rails_command is used for commands that require interactivity (:Console), commands that should be asynchronous (:Server), and commands that must be run inline to capture output (:Generate). This is iffy for the vagrant ssh case (:Console would require ssh -t, for example), and sounds like it would outright break things for an asynchronous FIFO. Open to other options but a flat configuration option seems like a no go.