Switch from structopt to clap

[Celeo]

Fixes #140.

Description

Replaces structopt with clap.

Without any additional formatting changes, this is how the help output is now written:

configuration for AWS S3 SDK

Usage: parseable [OPTIONS]

Options:
      --s3-endpoint-url <S3_ENDPOINT_URL>
          The endpoint to AWS S3 or compatible object storage platform [env: P_S3_URL=] [default: https://minio.parseable.io:9000]
      --s3-access-key-id <S3_ACCESS_KEY_ID>
          The access key for AWS S3 or compatible object storage platform [env: P_S3_ACCESS_KEY=] [default: minioadmin]
      --s3-secret-key <S3_SECRET_KEY>
          The secret key for AWS S3 or compatible object storage platform [env: P_S3_SECRET_KEY=] [default: minioadmin]
      --s3-default-region <S3_DEFAULT_REGION>
          The region for AWS S3 or compatible object storage platform [env: P_S3_REGION=] [default: us-east-1]
      --s3-bucket-name <S3_BUCKET_NAME>
          The AWS S3 or compatible object storage bucket to be used for storage [env: P_S3_BUCKET=] [default: parseable]
  -h, --help
          Print help information

---

This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

[nitisht]

Thank you for the PR, @Celeo we'll review and take it soon.

[trueleo]

Thank you for your kind work. 🎉