Code2prompt should have a Configuration File

[ODAncona]

This PR implements .code2promptconfig file

closes #209 and #101

Change 1

Code2Prompt default output is now:

👍 code2prompt . → stdout
📋 code2prompt . -c → clipboard
💾 code2prompt . -O file → file

Change 2

Code2Prompt now looks for a .c2pconfig file example:

default_output = "stdout"
include_patterns = ["*.rs"]
exclude_patterns = ["**/test*"]
line_numbers = false
absolute_path = true

[user_variables]
project = "code2prompt"