Fix error in SingleLinePerSelector for function selectors

The `SingleLinePerSelector` linter would throw an error for selectors
that contained a function call, e.g.

  #{foo()} {
    ...
  }

After a brief discussion with @sds, we decided to turn the opt-out step
of this linter into a whitelist instead of a blacklist. And now we only
run the linter on selectors that are all strings.

Fixes #199.

Change-Id: I261129871474788d58f981f09f6bf45366dad089
Reviewed-on: http://gerrit.causes.com/41002
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>