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.

llex.h

@@ -81,8 +81,9 @@ typedef struct LexState {
   TString *brkn;  /* "break" name (used as a label) */
   TString *glbn;  /* "global" name (when not a reserved word) */
   TString *cmd_envvar;       /* envvar name for $NAME in commands */
-  lu_byte cmd_mode;  /* 0=normal, 1=backtick command, 2=interactive command */
+  lu_byte cmd_mode;  /* 0=normal, 1=backtick, 2=interactive, 3=subcommand */
   lu_byte saved_cmd_mode;  /* cmd_mode to restore after interpolation */
+  lu_byte cmd_subcmd;  /* 1 when current interpolation is $() subcommand */
 } LexState;