Add tac utility

nl.nim

@@ -1,11 +1,26 @@
 import cligen
 import common
 
-proc main(files:seq[string], lineIncrement=1) =
+proc digits(num:int): int =
+    result = 1
+    var x = num
+    while x div 10 >= 1:
+        inc result
+        x = x div 10
+
+proc spaces(num:int): string =
+    for i in 1..num:
+        result &= " "
+
+proc formatNumber(num:int, width:int): string =
+    result = if digits(num) > width: $num
+             else: spaces(width - digits(num)) & $num
+
+proc main(files:seq[string], lineIncrement=1, numberSeparator="\t", numberWidth=6) =
+    var lineNr = 0
     for file in files:
-        var lineNr = 0
         for line in lines(file):
             lineNr += lineIncrement
-            writeLine(stdout, $(lineNr) & line)
+            writeLine(stdout, formatNumber(lineNr, numberWidth) & numberSeparator & line)
 
 dispatch(main, version=("version", nimbaseVersion))