feat(modules): add "register" module type

Inspired by https://github.com/ModuleLoader/es6-module-loader/wiki/System.register-Explained
Merge pull request #209 from douglasduteil/feat-modules-add-register-module-type

Add "register" module formatter
fix `util.regexify` on falsy values - fixes #208
fix explicit arguments reference for rest parameters - fixes #210
export as `module.exports` if there are no other ExportDeclarations in commonInterop module formatter @guybedford
clean up register module formatter and rename it to system
add 1.13.11 changelog
remove unused variables
fix util.regexify tests
v1.13.11
update modules docs
system tests updates for live bindings and function hoisting
ignore XJSEmptyExpression - fixes #214
add types.isDynamic so we can scan a node to see if it's ~safe~ to include it twice
Merge branch 'system' of github.com:guybedford/6to5 into guybedford-system
Merge branch 'guybedford-system'
Remove --no-timeouts flag.

Not a valid flag, causes errors when trying to use debug modes.
add 1.13.12 changelog
add VirtualPropertyExpression and SpreadProperty to ast-types
simplify generator context id