Add support for Button Element to contain a URL

[a1cooke]

Firstly, thanks for this great plugin, it is helping me alot.

Changes made

• Added url field to Element model in order to support type:button with a URL
• Slack documentation: https://api.slack.com/reference/block-kit/block-elements

Tests made

• Ran locally and confirmed behaviour
• Ran tests and are passing

This fixes an issue I created #8

[jongyoul]

Thx!! I'll check it sooner and publish a new version

[a1cooke]

Thanks @jongyoul , let me know when you plan to merge this