koa-timeout doesn't respect status parameter

[quarhodron]

koa-timeout package doesn't respect the status parameter.

I think in this place https://github.com/uswitch/koa-core/blob/master/packages/koa-timeout/koa-timeout.js#L19 it should be ctx.status instead of 408. Additionally, it would be nice if the error message was be parameterized.

pinging the last contributor @phelma