Add tool decorator to automatically document tools

[rlouf]

Solves #62

import outlines

@outlines.tool
def repeat(word: str, n: int):
    """Tool description."""
    return word * n

@text.prompt
def my_prompt(fn):
    """COMMAND: {{fn.name}}, {{fn.description}}, args: {{fn.args}}"""

my_prompt(repeat)
# COMMAND: repeat, Tool description, arg: word: str, n:int