Change --size and --margin of encode command

[sorairolake]

Description

Change these to optional arguments and remove the default values.

• The default value of the module size for UTF-8 string is preferably 1, otherwise 8 is preferred¹².
• The default value of the margin for Micro QR code is preferably 2, otherwise 4 is preferred³⁴.

Closes #

Additional context

Checklist

• I have read the Contribution Guide.
• I agree to follow the Code of Conduct.

Footnotes

1. https://docs.rs/qrcode/0.13.0/src/qrcode/render/unicode.rs.html#20 ↩

2. https://docs.rs/qrcode/0.13.0/src/qrcode/render/mod.rs.html#26 ↩

3. https://docs.rs/qrcode/0.13.0/src/qrcode/lib.rs.html#250 ↩

4. qrencode --help ↩