[feature] Allow more general namespacing

[siefkenj]

The proposal is to add a namespace_path attribute that would accept a syn::Path. The generated type would then be wrapped in the specified namespace (with :: indicating levels of the namespace).

For example:

#[derive(Tsify)]
#[tsify(namespace_path = My::Path)]
struct Foo { a: i32 }

would produce

declare namespace My {
  declare namespace Path {
    interface Foo { a: number }
  }
}

There should be a corresponding option to use a path as a namespace for a declared type. For example

#[declare]
#[tsify(path_as_namespace)]
type Foo = my::path::X;

would produce

export type Foo = my.path.X;

This would give a way to handle name collisions, addressing #16

Not sure what the best name for the attrs should be. It also doesn't follow Typescripts convention of using CamelCase to name namespaces, but a compromise needs to be made somewhere...