Make ask prompt configurable

[ldoguin]

Add a configurable prompt for each model in the ask command, add example under examples/writer-agent. This would enable ai agents creation with cbsh.

Here is an example that specify answers must always be in JSON format:

👤 Laurent Doguin 🏠 capella in ☁️ cbsh.commits._default
> ask --prompt "you are a helpful bot, always answering in the JSON text format" "list all films by Steven Spielberg with their name, release date and main cast"
{
  "films": [
    {
      "name": "Jaws",
      "release_date": "1975",
      "main_cast": ["Roy Scheider", "Robert Shaw", "Richard Dreyfuss"]
    },
    {
      "name": "E.T. the Extra-Terrestrial",
      "release_date": "1982",
      "main_cast": ["Henry Thomas", "Drew Barrymore", "Dee Wallace"]
    },
    {
      "name": "Jurassic Park",
      "release_date": "1993",
      "main_cast": ["Sam Neill", "Laura Dern", "Jeff Goldblum"]
    },
    {
      "name": "Schindler's List",
      "release_date": "1993",
      "main_cast": ["Liam Neeson", "Ben Kingsley", "Ralph Fiennes"]
    },
    {
      "name": "Saving Private Ryan",
      "release_date": "1998",
      "main_cast": ["Tom Hanks", "Matt Damon", "Tom Sizemore"]
    },
    {
      "name": "Catch Me If You Can",
      "release_date": "2002",
      "main_cast": ["Leonardo DiCaprio", "Tom Hanks", "Christopher Walken"]
    },
    {
      "name": "The Terminal",
      "release_date": "2004",
      "main_cast": ["Tom Hanks", "Catherine Zeta-Jones", "Stanley Tucci"]
    }
  ]
}