Another option is to use duti (<https://github.com/moretension/duti>). 

Run `brew install duti`, save a filetype like this as:

    duti -s com.sublimetext.3 public.plain-text all

The changes should be applied immediately, so you don't have to restart like when editing `com.apple.LaunchServices.plist`.

To also change the default application for executable scripts with no filename extension, add a line like this:

    duti -s com.sublimetext.3 public.unix-executable all

Some files are also considered 'public.data', not 'public.plain-text', so you can do this as well:

    duti -s com.sublimetext.3 public.data all