Add option to open .csv files from the Files pane using Excel or OS default .csv app

[jzadra]

Currently when opening files from the Files pane using "View File", excel files will open in excel, but .csv files open in a plain text source window.

It is often very difficult to browse the data in this format, especially for larger data frames with many columns. It would be nice if there was an option to open .csv's in excel, or better yet, in the OS system default registered app for .csv's.

[ronblum]

@jzadra Thank you for the suggestion! We'll review this as we continue development of RStudio.