Add a history size parameter to the chat

If too many messages are used in the prompt, the model
gets really slow. It is useful to have the ability to
limit this.

settings-template.json

@@ -9,6 +9,9 @@
     "prompt": "Common sense questions and answers\n\nQuestion: \nFactual answer:",
     "prompt_gpt4chan": "-----\n--- 865467536\nInput text\n--- 865467537\n",
     "stop_at_newline": true,
+    "history_size": 8,
+    "history_size_min": 0,
+    "history_size_max": 64,
     "preset_pygmalion": "Pygmalion",
     "name1_pygmalion": "You",
     "name2_pygmalion": "Kawaii",