Allow setting store options

[nalenz-objectbox]

Using obx_opt_* functions, properties like the store path, database filename etc. can be set. See here for the list of possible options.

Use a Store constructor with named arguments to easily specify any non-default options.

[greenrobot]

Should we have a StoreBuilder class for that? Are builders common in Dart?

[nalenz-objectbox]

I wouldn't go for a separate builder class. Doing something like new Store(model, directory: "mydir", maxDBSizeInKB: 524288) is quite intuitive already I think.

[greenrobot]

"Named optional parameters"