Support to set default emoji

[umisora]

About

I want to change the emoji because default emoji is not cool.

Change to

Changed lambda to accept env of default_emoji.

How to use

Specify default_emoji in your terraform code

module "sns_to_slack" {
  source = "github.com/xxxxxxx/aws-sns-slack-terraform/module"

  slack_webhook_url = "hooks.slack.com/services/xxxxxxxx/xxxxxxxx"
  slack_channel_map = "{ \"topic-name\": \"#slack-channel\" }"
  default_emoji     = ":tada:"  ← New
}

[umisora]

Sample Slack UI ( in my project )

[umisora]

@builtinnya
Thank you for your aws-sns-slack-terraform.
Please review this PR.

[builtinnya]

Thank you for your PR!
LGTM 👍

[umisora]

Thank you for your review!