Add a --flavour option to the create command

[raphaelstolt]

Add a --flavour option to the create command which then either creates a .gitattributes file with classic or negated export-ignore directives.

Example of a .gitattributes file with negated export-ignore directives:

# Line-ending normalization for text files.
* text=auto

# PHP-aware hunk headers in `git diff`.
*.php diff=php

# Binary files keep their bytes.
*.gpg binary
*.phar binary

# Default: nothing is included in the dist archive of the Composer package.
# Re-include only the files that actually belong to the distributed package below.
* export-ignore

# Top-level metadata.
composer.json  -export-ignore
LICENSE.md     -export-ignore

# Source code and CLI entry point.
/src           -export-ignore
/bin           -export-ignore