add alpha-numeric support (full ASCII charset, 32-127)

via 15-segment display tube (similar to Burroughs B-7971).

README.md

@@ -1,12 +1,30 @@
 nixie_tubes mod by Vanessa Ezekowitz
 
-This mod provides a set of classic Nixie tubes, controlled by Mesecons' 
+This mod provides a set of classic Nixie tubes, and a set of alphanumeric
+15-segment tubes similar to Burroughs B-7971, controlled by Mesecons'
 Digilines mod.
 
 Simply place a tube, right-click it, and set a channel.
 
-Then send a message to that channel from a Mesecons Lua Controller with either
-a number 0-9, the word "colon", the word "period" or the word "off".  The tube
-will "light-up" the appropriate number or symbol, or turn off entirely.
+Then send a character or one of several control words to that channel from a
+Mesecons Lua Controller and the tube will try to display it.
+
+The classic tubes are numeric with colon and period symbols, and hence will
+respond to the literal numbers 0-9, and the words "colon", "period", and
+"off".  Any other symbol or word is ignored.
+
+The alphanumeric tubes respond to characters from the standard 7-bit ASCII
+character set, along with these messages:
+
+* "off", "colon" and "period" act the same as on the numeric tubes.  Note that
+  neither a colon nor a period actually look all that great on a 15-segment
+  display, so use a classic tube for those, if you can.
+* "del" or character code 127 displays an all-on square, but without segment
+  #15 (the bottom, chevron-shaped one).
+* "allon" or character code 128 will display an all-on square, with segment
+  #15 lit also.
+* "cursor" or character code 129 will display just segment 15.
+
+Any unrecognized word or symbol outside the 32-129 range is ignored.
 
 Tubes emit a small amount of light when displaying something.