Allow option for each snippet as to whether it is expanded when spacebar, rather than tab is pressed

[vjpr]

E.g.

a is<press-spacebar> b

becomes:

a === b

In the snippets.cson:

  '===':
    'prefix': 'is'
    'body': '=== '
    'activation-key': 'space'

This is similar functionality offered by IntelliJ Live Templates.