feat: allow set router host match

[yeliex]

If a server with many hosts and the same path in each host may different.

current version:

router.get('/', (ctx) => {
    if(ctx.host === 'a') // ...
    if(ctx.host === 'b') // ...
})

it is a good way to support host match in router definition:

// hosta.router.js
const routerA = new Router({
  host: 'hosta.com' // only match if request host exactly equal `hosta.com`
});
router.get('/', (ctx, next) => {
  // Response for hosta.com
});

// hostb.router.js
const routerB = new Router({
  host: /^(.*\.)?hostb\.com$/ // match all subdomains of hostb.com, including hostb.com, www.hostb.com, etc.
});
router.get('/', (ctx, next) => {
  // Response index for matched hosts
});

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• [ x I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

[yeliex]

@titanism @etroynov pls

[titanism]

v11.0.1 released

https://github.com/koajs/router/releases/tag/v11.0.1