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.

lcmd.h

@@ -14,11 +14,13 @@
 #define LUSH_OP_INTERACTIVE  1
 #define LUSH_OP_GETENV       2
 #define LUSH_OP_SETENV       3
-#define LUSH_OP_COUNT        4
+#define LUSH_OP_SUBCMD       4
+#define LUSH_OP_COUNT        5
 
 int lushCmd_command (lua_State *L);
 int lushCmd_interactive (lua_State *L);
 int lushCmd_getenv (lua_State *L);
 int lushCmd_setenv (lua_State *L);
+int lushCmd_subcmd (lua_State *L);
 
 #endif