Add "Access-Control-Allow-Methods" to the OPTIONS HTTP method

## Feature Request

https://github.com/rust-net-web/tide/pull/104 introduces the `OPTIONS` HTTP method for CORS support. In order to get it to work, the `Access-Control-Allow-Methods` header has been omitted for now.

## Detailed Description
We should reintroduce `Access-Control-Allow-Methods` through its own CORS middleware.

## Context
This is a nice addition to our CORS capabilities.

## Possible Implementation
See https://github.com/rust-net-web/tide/pull/104#discussion_r246828800 for a discussion on what was difficult in adding this. As per https://github.com/rust-net-web/tide/pull/104#discussion_r238703325 it's been suggested this should probably be its own middleware