Implement --script command line option to parse as Script.

Option --script followed by name of a file to be parsed as Script.
Default changed: now files are parsed as Module.
Test changes to verify values via globlals (modules have no value).
Module compilation simulates System.import() but post-pending
 System.get(normalizedName) to the output.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/760

Closes #760.