Add train/2 and add_weight/4 implementations to Markov Chain

* train/2 takes a chain and a list of words to train them on
* add_weight/4 takes a chain, a list of words (the key), the word it
  points to, and an optional increment which will increment the value of
  a weight, or insert the weight.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

test/contrib/markov/chain_test.exs

@@ -2,11 +2,32 @@ defmodule MarkovChainTest do
   use ExUnit.Case
   alias Omnibot.Contrib.Markov.Chain
 
-  test "chain train_one works correctly" do
+  test "chain train works correctly" do
     chain = %Chain {order: 2}
-            |> Chain.train_one(["foo", "bar"], "baz")
-    #assert chain.chain == [
-      #{["foo", "bar"], {"baz", 1}}
-      #]
+            |> Chain.train(~w(foo bar baz))
+    assert chain.chain == [
+      {["bar", "baz"], %{nil => 1}},
+      {["foo", "bar"], %{"baz" => 1}},
+    ]
+  end
+
+  test "chain add_weight works correctly" do
+    chain = %Chain {order: 2}
+            |> Chain.add_weight(["foo", "bar"], "baz")
+    assert chain.chain == [
+      {["foo", "bar"], %{"baz" => 1}}
+    ]
+
+    chain = chain |> Chain.add_weight(["foo", "bar"], "baz", 2)
+
+    assert chain.chain == [
+      {["foo", "bar"], %{"baz" => 3}}
+    ]
+
+    chain = chain |> Chain.add_weight(["foo", "bar"], "qux")
+
+    assert chain.chain == [
+      {["foo", "bar"], %{"baz" => 3, "qux" => 1}}
+    ]
   end
 end