Add lua scripts for generating n-grams from SKPs

tools/lua/ngrams_aggregate.lua

+-- Aggregate the output from ngrams.lua.
+
+-- Get the data from all shards.
+counts = {}
+dofile("/tmp/lua-output")
+
+-- Put the data into a sortable "array".
+countArray = {}
+for ngram, count in pairs(counts) do
+  table.insert(countArray, {count, ngram})
+end
+
+-- Sort the data.
+function compare(a, b)
+  return a[1] > b[1]
+end
+table.sort(countArray, compare)
+
+-- Write the result.
+for i, countPair in ipairs(countArray) do
+  io.write(countPair[1], "\t", countPair[2], "\n")
+end