Add command line option --module <module-name>.

  Allows scripts and modules to be compiled in any order.

Review URL: https://github.com/google/traceur-compiler/pull/1064

Closes #1064.
Only add System.get() evaluator for modules=register.

Fixes #1066

Review URL: https://github.com/google/traceur-compiler/pull/1067

Closes #1067, closes #1066.
Destructuring and cover grammar fixes

This changes the strategy from using a transformer to transform the
cover grammars into rewinding the scanner and reparsing using the
target grammar.

This also introduces a AssignmentElement which is used for the
elements of the assignment pattern (like BindingElement which is used
for binding patterns (and default parameters).

Fixes #183
Fixes #836
Fixes #838
Fixes #969
Fixes #1015