Provide separate modules for different Caddy API endpoints/objects

[maxhoesel]

As discussed in #7, it would be nice if this collection had purpose-built modules for modifying certain parts of the Caddy API, such as the ACME config or http servers. These modules could use the same basic framework as the existing caddy_config/load modules and would exist in parallel to the old modules, allowing users easy modifications to commonly-used objects, while still leaving the option for more advanced modifications using caddy_config.

Questions that still need to be answered:

• What "endpoints" do we want to support?
• How much effort is it to implement these modules, and is the benefit for the user big enough to justify it?
• Do we want to do any validation on the passed data, or should we lave that to Caddy?
• How do we plan to keep up with upstream changes to Caddys API?
• Who's going to write the code? :)