Add ability to change rspec run command after opening vim

This pull request allows changing of the rspec_command after vim is started. In my case this is useful to be able to toggle whether vim-rspec should use spring via this command:

```
:command Spring :call SpringToggle()
function! SpringToggle()
  let s:current_command = GetSpecCommand()
  if s:current_command == s:springon
    call SetSpecCommand(s:springoff)
    echo "Spring disabled"
  else
    call SetSpecCommand(s:springon)
    echo "Spring enabled"
  endif
endfunction
```