Translate dashes to underscores in API names

so that `--backend openai-chatgpt` works like `--backend openai_chatgpt`

README.md

@@ -98,8 +98,8 @@ an existing session with `-s`. Or, you can continue the last session with
 You can set the "system message" with the `-S` flag.
 
 You can select the text generating backend with the `-b` flag:
- * openai\_chatgpt: the default, paid API, best quality results
- * llama\_cpp: Works with [llama.cpp's http server](https://github.com/ggerganov/llama.cpp/blob/master/examples/server/README.md) and can run locally with various models,
+ * openai-chatgpt: the default, paid API, best quality results
+ * llama-cpp: Works with [llama.cpp's http server](https://github.com/ggerganov/llama.cpp/blob/master/examples/server/README.md) and can run locally with various models,
  though it is [optimized for models that use the llama2-style prompting](https://huggingface.co/blog/llama2#how-to-prompt-llama-2).
  Set the server URL with `-B url:...`.
  * textgen: Works with https://github.com/oobabooga/text-generation-webui and can run locally with various models.

src/chap/core.py

@@ -55,6 +55,7 @@ def configure_api_from_environment(api_name, api):
 
 
 def get_api(name="openai_chatgpt"):
+    name = name.replace("-", "_")
     result = importlib.import_module(f"{__package__}.backends.{name}").factory()
     configure_api_from_environment(name, result)
     return result