Display a message with the number of detected workers

[gibachan]

As described in #103, This PR adds a message the number of workers before processing them.

You can see the message like below.

% cargo build
% cd examples/
% ../target/debug/wws .
⚙️  8 files are identified as workers. Start processing them
⚙️  Loading routes from: .
🗺  Detected routes:
    - http://127.0.0.1:8080/js-redirect/handler
      => ./js-redirect/handler.js (name: default)
    - http://127.0.0.1:8080/js-json/handler
      => ./js-json/handler.js (name: test)
    - http://127.0.0.1:8080/js-basic
      => ./js-basic/index.js (name: default)
    - http://127.0.0.1:8080/js-tictactoe/handler
      => ./js-tictactoe/handler.js (name: tictactoe)
    - http://127.0.0.1:8080/js-params/[id]
      => ./js-params/[id].js (name: default)
    - http://127.0.0.1:8080/js-params/fixed
      => ./js-params/fixed.js (name: default)
    - http://127.0.0.1:8080/js-params/sub/[id]
      => ./js-params/sub/[id].js (name: default)
    - http://127.0.0.1:8080/js-params/[id]/fixed
      => ./js-params/[id]/fixed.js (name: default)
🚀 Start serving requests at http://127.0.0.1:8080

resolve #103

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