Add prompt templates for formats so users don't have to constrain the output

[JasonWeill]

Problem

When users customize the output format using the -f/--format option to the magic command, the prompt may still cause the AI to output in a format that the magic command can't display. For example, if the user asks for HTML markup, the AI model may generate explanatory text that is not in HTML format.

Proposed Solution

For each supported output format, write a prompt template. Apply the prompt template to the specified prompt before sending it to the AI model.

In the examples below, the prompt template is indicated in bold.

%%ai anthropic:claude-v1.2 -f html
Create a square using SVG with a black border and white fill.

… would send:

Create a square using SVG with a black border and white fill. The output should be in HTML format only, with no text before or after.

%%ai chatgpt -f math
Generate the 2D heat equation

… would send:

Generate the 2D heat equation

The output should be in LaTeX surrounded by $$. Do not include an explanation.

[JasonWeill]

Fixed by #53.