Conceptually the backend configuration should be validated when creating
or opening the backend, but not when filling in information from
environment variables into the configuration.
In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.
An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.
The `s3.storage-class` option can be passed to restic (using `-o`) to
specify the storage class to be used for S3 objects created by restic.
The storage class is passed as-is to S3, so it needs to be understood by
the API. On AWS, it can be one of `STANDARD`, `STANDARD_IA`,
`ONEZONE_IA`, `INTELLIGENT_TIERING` and `REDUCED_REDUNDANCY`. If
unspecified, the default storage class is used (`STANDARD` on AWS).
You can mix storage classes in the same bucket, and the setting isn't
stored in the restic repository, so be sure to specify it with each
command that writes to S3.
Closes #706
