rainbow-dark: drop rainbow dashes, add rainbow-timestamp.pl

- rainbow-dark.theme: revert line_start back to the stock '-!-' pattern.
- .irssi/scripts/autorun/rainbow-timestamp.pl: new autorun script that
  hooks 'print text', matches a leading HH:MM[:SS], and reruns it
  through rainbow.pl's make_colors (same palette [4,8,9,11,12,13],
  same 2-char stride, same phase-advance on each line). The result
  is that every successive timestamp is a 5-character rainbow
  gradient that also shifts starting colour line-to-line.

.irssi/rainbow-dark.theme

@@ -62,11 +62,8 @@ abstracts = {
   ## generic
   ##
 
-  # text to insert at the beginning of each non-message line — ROYGBIV dashes
-  #   R = bright red, O = dark yellow/brown (closest ANSI orange),
-  #   Y = bright yellow, G = bright green, B = bright blue,
-  #   I = dark blue (closest ANSI indigo), V = bright magenta (violet).
-  line_start = "%R-%y-%Y-%G-%B-%b-%M-%n ";
+  # text to insert at the beginning of each non-message line (default)
+  line_start = "%B-%n!%B-%n ";
 
   # timestamp styling, nothing by default
   timestamp = "$*";