support mapping specific java classes to custom TypeScript types

[atsu85]

The library should provide a hook to support mapping application-specific classes to custom TypeScript types. One common use-case, could be for instance Joda or Java8 LocalDate or Date or Calendar to TypeScript/JavaScript Date

[atsu85]

please merge my pull request to resolve this issue

[atsu85]

pull request contains test for this feature, that basically

1. creates configuration object with type mapping from CustomDate.class to Date (represented by DateType class)
2. creates in-memory representation of TypeScript module with the configuration and one TestClass
3. writes module to StringWriter
4. cheks that generated output is equal to expected output (this test uses external module format instead of internal module format)