Fix: wrong addDatabaseType() signature

[reasv]

The static method OrbitDB.addDatabaseType(type: string, store: Store) has the wrong type signature and that makes it impossible to use, as it will always cause a compile error even when used correctly:

src/index.ts:30:38 - error TS2345: Argument of type 'typeof Store' is not assignable to parameter of type 'Store'.
  Type 'typeof Store' is missing the following properties from type 'Store': address, all, type, key, and 6 more.

30     OrbitDB.addDatabaseType('store', Store)
                                        ~~~~~

  src/index.ts:30:38
    30     OrbitDB.addDatabaseType('store', Store)
                                            ~~~~~
    Did you mean to use 'new' with this expression?

Found 1 error.

The issue is very subtle, but store: Store actually indicates that the function accepts an instance of Store. In reality we want the function to take a class, whether that's Store, or something that inherits from it. I fixed it by changing the signature to

static addDatabaseType(type: string, store: typeof Store);

[RichardLitt]

Thanks, @reasv