Implement $(cmd) subcommand syntax in commands (issue #27)

Add $() for inline subcommand substitution that runs a command and
inserts its stdout (trailing newlines stripped) into the outer command
string. Supports nesting, and mixing with ${expr} and $NAME.

issues/27-subcommand-syntax.md

@@ -1,6 +1,6 @@
 # 27 — Subcommand syntax in commands
 
-**Status:** open
+**Status:** done
 
 ## Problem