Extended `EXPOSE` to allow port ranges

- Example: `EXPOSE 8080-8089` opens ports 8080 to 8089 (inclusive)
- Valid port ranges should be in form [0-9]+-[0-9]+
- buildfile_test.go tests edge values as well as an 'implied' inner
  value
- Note: Ports are not confirmed to within 1 and 65535 when parsed.
- Note: Ports are not checked for uniqueness when parsed.