* Add webhook url overload for DiscordWebhookClient
Adds an overloaded constructor for `DiscordWebhookClient` which accepts the webhook URL. This URL is parsed using a regex for the id and token. If the token is invalid an `ArgumentException` is thrown.
* add null or whitespace check
* add some tests for the new DiscordWebhookClient constructor
* make the Regex static, specify flags
* update regex to look for "discordapp"
* specify reason why exception is thrown despite regex match
* move parsing logic into new function for testing
Chris Johnston