Extend fixture label replacement to allow string interpolation

Allows fixtures to use their $LABEL as part of a string instead of limiting use to the entire value.

    mark:
      first_name: $LABEL
      username: $LABEL1973
      email: $LABEL@$LABELmail.com

    users(:mark).first_name # => mark
    users(:mark).username   # => mark1973
    users(:mark).email      # => mark@markmail.com