Support composite key for PrimaryKey decorator

[dmitryrazinkov]

@TiBianMod Hi! Thank you for reviewing my previous PR and for your work on the library.

I updated it, added tests and updated the docs. Please be free to reject it if you believe that it's not needed for the library and having the possibility to set primaryKey static property is enough.

• In vuex-orm you can define a composite primary key as static primaryKey = ['userId', 'voteId'], so added support for that

• Usage:

  {
  @PrimaryKey() @StringField() public userId: string
  
  @PrimaryKey() @StringField() public voteId: string 
  }

[TiBianMod]

Thank you for the pull request @dmitryrazinkov