Mark Config options as optional in TypeScript typings

[pe8ter]

With strict typings enabled, consumers are forced to specify all fields for the Config object, even though configureInstaller(conf) already handles missing fields.

Right now I have to do this

DynamoDbLocal.configureInstaller({
    installPath: '/path/to/my/tmp',
    downloadUrl: null
});

when I should be able to do this

DynamoDbLocal.configureInstaller({
    installPath: '/path/to/my/tmp'
});

[rynop]

v0.0.31 published. Thanks!

[pe8ter]

No prob. Thanks for the fast turnaround!