Add tac utility

tac.nim

@@ -0,0 +1,34 @@
+import cligen, os, streams
+import common
+
+proc catFile(file:string) =
+    if not existsFile(file):
+        err "tac: " & file & ": No such file or directory"
+        system.quit 1
+    var
+        f = open(file, fmRead)
+        reverseLines: seq[string]
+    for line in lines(f):
+        reverseLines = line & reverseLines
+    for line in reverseLines:
+        writeLine(stdout, line)
+
+proc catStdin() =
+    var
+        line = ""
+        reverseLines: seq[string]
+    while stdin.readLine(line):
+        reverseLines = line & reverseLines
+    for line in reverseLines:
+        writeLine(stdout, line)
+
+proc main(files:seq[string]) =
+    if files.len == 0:
+        catStdin()
+    for file in files:
+        if file == "-":
+            catStdin()
+        else:
+            catFile(file)
+
+dispatch(main, version=("version", nimbaseVersion))