Configurable transforms

[dejang]

This PR addresses #34 and adds configurable options for internal source parsers used by realm-shim.

rejectImportExpressions: true|false : default true, use the rejectImportExpressions transformer
rejectHtmlComments: true|false : default true, use the rejectHtmlComments transformer
rejectSomeDirectEvalExpressions: true|false : default true, use the rejectSomeDirectEvalExpressions transformer

Set flags to false to selectively disable mandatory transformers. By default all transformer are enabled.

// a compartment which has only rejectHtmlComments enabled
Realm.createCompartment({ rejectImportExpressions: false, rejectSomeDirectEvalExpressions: false }) 

// a root realm having only rejectImportExpressions and htmlComments enabled
Realm.makeRootRealm({ rejectSomeDirectEvalExpressions: false })

[caridy]

I will like to see this as part of the factory() rather than as new configs at the compartment level.