Add `content:insert` and `content:remove` hooks

[Philipp Daun]

Describe the problem

• There's currently no way of accessing the new container before it's inserted into the dom
• This makes debugging and adding initial styles to containers harder than necessary
• Discussed in #8

Describe the proposed solution

• Create a new hook content:insert
• Create a new hook content:remove
• Or maybe content:enter and content:leave

Alternatives considered

• Modify the HTML with initial styles

How important is this feature to you?

• Nice to have
• Would make my life a lot easier
• I cannot use swup without it