better export modules as library

[Kanahiro]

to use chiitiler as library, following snipets are needed

import { initServer, cache } from 'chiitiler';

const { tiles } = initServer({
    cache: cache.s3Cache({
        bucket: 'bucketname',
        region: 'ap-northeast-1',
        endpoint: null,  // needed in special cases
    }),
    port: 3000, // unused
    debug: false, // unused
});

initServer has been called only by CLI but now called as library.
As library, the signature looks somewhat awkward.