Add option "url_safe" to "to_base64" function

[dr0i]

Resolves #384.

[dr0i]

Other than suggested in #384 we cannot use an option with hyphen, like the suggested url-safe. Options can use the underscore _, though.
Not sure if we want to use camel case (urlSafe? safeUrl?) or safe_url instead of url_safe ?

[TobiasNx]

Other than suggested in #384 we cannot use an option with hyphen, like the suggested url-safe. Options can use the underscore _, though. Not sure if we want to use camel case (urlSafe? safeUrl?) or safe_url instead of url_safe ?

url_safe as stated here: https://www.base64encode.org/