HTTP cache middleware for midd

$3

``js
const middCache = require('midd-cache');
app.use(middCache())
`

#### Manually check cache fresh when request coming
After cache expired in the client(browser), client will send a revalidate request with If-Modified-Since or If-None-Match headers.
Use req.fresh(status) to check that, and if cache is fresh, it will response 304 and return true, otherwise it will return false.
The status is current resource state about Last-Modified or ETag.
It could have two keys(keys name is case insensitive):
* Last-Modified or LastModified: number(ms) or string(utc) or Date. The string format is like date.toUTCString()
* ETag: the etag string

Example:
`js
app.use((req, resp, next)=>{
let lastMod = getRecordLastModified(req.params.id);
if(req.fresh({'Last-Modified': lastMod})) return;
resp.send(updatedRecord)
})
`

#### Auto check cache fresh when using resp.send
This mechanism must be used with midd-send middleware and only effective on resp.send method. You should attch midd-send first.
When you call resp.send(body) method, it will generate ETag from the body, and get Last-Modified from the headers.
Then use them to compare with the request's If-Modified-Since or If-None-Match. If the cache is fresh, respond 304, otherwise respond the body with ETag and Last-Modified headers.

#### Set cache headers for response
resp.setCache(options)
Options have:
* maxAge: number, milliseconds. Used in Cache-Control header
* private: boolean. Used in Cache-Control header
* Last-Modified or LastModified: number(ms) or string(utc) or Date. The key name is case insensitive.
* ETag`: the etag string. The key name is case insensitive.

$3

Licensed under MIT

Copyright (c) 2017 Tian Jian