Add auto-execution for chained requests

This is a breaking change because the !request variant of ChainSource is now a map instead of just a string. I also converted the other variants of ChainSource to be maps as well, for consistency/future expansion.

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 ## [Unreleased] - ReleaseDate
 
+### Breaking
+
+- All variants of the `Chain.source` field are now maps
+  - This is to support the next request auto-execution feature, as well as future proofing for additional chain configuration
+
+### Added
+
+- Chained requests can now be auto-executed according to various criteria. See [the docs](https://slumber.lucaspickering.me/book/user_guide/chaining_requests.html) for more
+
 ### Changed
 
 - Don't print full stack trace for failed CLI commands