Discord-webhook

Features
- add optional `rate_limit_retry` to webhook.
The webhook will automatically be sent once the rate limit has been lifted

Features
- add `timeout` param to webhooks

Features
- convert embed color to decimal if it's given as hex
- add `remove_embeds()`, `remove_files()` functions in order to clear `embeds` and `files` properties of webhook object
- add optional `remove_embeds` and `remove_files` args to `execute()` in order to automatically clear `embeds` and `files` properties
- add `remove_file()` function in order to remove a file in `files` given by filename

Breaking Changes
- return webhook responses only as a list if multiple urls are given. Otherwise, just return the response object

- webhook responses are always returned as a list

Features
- `edit()` and `delete()` methods are now supporting multiple webhooks

Fixes
- fixed an issue where multiple webhook urls would throw an error after `.execute()` has been called

- add `edit()` and `delete()` methods to `DiscordWebhook` class

- add `allowed_mentions` property to webhooks. see [Discord Docs](https://discord.com/developers/docs/resources/channel#allowed-mentions-object)