Add index signatures for classes

[draconisNoctis]

Hey @dsherret,

first of all thanks for this awesome library 👍

Now to my problem/feature request: I've tried to add a index signature to a class, created with ts-morph, but the required API's are only implemented for interfaces.

Workaround: I've created an empty class (required for decorators), added an interface with the very same name and let them merge via the declaration merging of Typescript.

Thanks, Mark