Consider check_trailing_bits=false for BASE64_MIME decoder

[wathiede]

In staktrace/mailparse#96 it was considered to request this package allow trailing bits on the BASE64_MIME codec. The assumptions behind this reasoning are:

• BASE64_MIME is tailored for email
• in real world emails are sometimes imperfect, and returning something is better than nothing

Would this project be willing to accept a pull request that makes BASE64_MIME more liberal in its handling by setting check_trailing_bits=false? If not, that's okay, I can go back to the mailparse project and see if they're willing to handle it.

Note, I'm not associated with the project, I just personally stumbled across this problem recently, so I figured I'd start with their suggestion first.

[ia0]

There's at least 3 options:

• The solution in base64 decoding should not be "strict" staktrace/mailparse#96 which is to define their own encoding with lazy-static.
• Using data-encoding-macro which doesn't need lazy-initialization because it produces the encoding at compile-time (see below for a snippet).
• Add a BASE64_MIME_PERMISSIVE constant in data-encoding. (I'm not willing to change the behavior of BASE64_MIME because this would be a breaking change. But adding a new constant for those who need to ignore trailing bits is fine.)

const BASE64_MIME_PERMISSIVE: data_encoding::Encoding = data_encoding_macro::new_encoding! {
    symbols: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
    padding: '=',
    check_trailing_bits: false, // NEW (defaults to true)
    wrap_width: 76,
    wrap_separator: "\r\n",
};

What do you think? Happy to add BASE64_MIME_PERMISSIVE if that's what you would prefer.