Add Code Completion for R

Hi Jeff:

Here is my first attempt for #20. I had to upgrade ace library to 1.1.8 for some new features related to auto completion. I took the library from https://github.com/ajaxorg/ace-builds/tree/v1.1.8/src-min-noconflict. Hope this is okay for you.

For a quick summary, I have:

1.  Updated `aceEditor` to take `autoComplete` to enable/disable code completion and `autoCompleteList` to take list of static code completion candidates. Again the latter is very useful for NSE functions as you can try in the demo below.
2.  Updated `updateAceEditor` to allow enable/disable autocomplete and update static completion list.
3.  Added `aceAutocomplete` to invoke `utils:::.win32consoleCompletion` which delivers the same code completions experience as R console.

The demo included in examples can be seen at: https://saurfang.shinyapps.io/shinyAceAutocomplete/

You can review this commit for all my code changes: https://github.com/saurfang/shinyAce/commit/84d45dbecc82d9261e01d6425e70db9756824c28

Can you take a look and let me know if you have any questions or suggestions?