Allow diffAttrs to use brace expansion (includes PR)

[BillyRayPreachersSon]

I wasn't sure if I had to raise an issue to match my PR, so apologies if this is an unnecessary step.

Brace expansion will be familiar to anyone who has worked with Ember's computed properties, and it's something I've been wanting to use with ember-diff-attrs for a while.

It's currently unsupported, but I've raised PR #34 to allow it to work.

For those unfamiliar with brace exapansion, it allows developers to specify related property names grouped together in curly braces, to avoid having to specify the parent path multiple times.

Before:

didReceiveAttrs: diffAttrs('user.forename', 'user.age', 'admin.surname', 'admin.email', 'admin.age', function(changedAttrs) {
  ...
})

After:

didReceiveAttrs: diffAttrs('user.{forename,age}', 'admin.{surname,email,age}', function(changedAttrs) {
  ...
})

[BillyRayPreachersSon]

@workmanw Would you be OK to merge the PR behind this (#34)?