Simplify command line syntax of some options

[mountaineerbr]

This wrapper script started without a definitive interface. Many ideas were tried out and only the most convenient features survived.

The script itself is supposed to be backwards-compatible, even though some major refactoring of code were done, we strived to stabilise the user interface. For example, the history file has the same TSV structure almost since the beginning and the user interface received mostly improvements and very few changes.

However, some features were superimposed and the syntax of some options became too convoluted.

---

For example, to load a custom prompt without editing it, it was necessary to set command line with the option -..[prompt_name]. This was the syntax I mostly run and because once you have set up a custom prompt you probably don't wish to modify it every time, and as this syntax became tiresome to type, it was recently hard-aliased as the option -.[prompt_name].

Single-shot editing of a custom prompt was changed to option -,[prompt_name] and editing the actual custom prompt file template was changed to option -,,[prompt_name].

---

In release v0.81, we wish to change the defaults script mode when no command line option is set, meaning that running chatgpt.sh without options should open the native chat completion single-turn mode and no history support, instead of defaulting to the legacy text completion mode.

Mind that for text completion in multi-turn mode, the user can still set option -d as usual, but to run the script in single turn of pure text completion then, he needs to set a text completion model, such as chatgpt.sh -m gpt-3.5-turbo-instruct.

For multi-turn chat modes, options remain unchanged. Set option -c (text chat completions) or options -cc (native chat completions).

---

Change heeding envar API_HOST to BASE_URL.