'/' in vim mode maps to search

This pull request maps '/' to search. Issue raised in #2034. 
If you could help to implement showCommandLine that would be absolutely great! 
The idea with `showCommandLine` is that most of the things done by vim command lIne do not depend on ace, (commands like w,q, positioning commandline on the screen, etc.), and `showCommandLine` provides a hook for ides embedding ace to provide their own implementation.
Perhaps we can include with ace a simple implementation of [showCommandLine](https://github.com/ajaxorg/cloud9/blob/master/plugins-client/ext.vim/vim.js#L104-L110) that will display ace instance with fixed position at the top of the page and will handle search commands.
But I am not really familiar with vim, so this whole approach might be wrong. If you know vim better and have a better idea about supporting this please tell.