Harmonize how modifiers are passed

- ~~Rename `container` in to `inner`. Can be done quickly after #1229 is merged.~~ Done in #1243
- Harmonize the way the modifiers can be passed (as `Nested` parameters, as Meta params, at load time, in schema init, as string with dot delimiter, etc.).
- Merge `Schema._normalize_nested_options` and `Nested._nested_normalized_option` into a single method.

_Originally posted by @lafrech in https://github.com/marshmallow-code/marshmallow/pull/1066#issuecomment-498558303_

See also:

- https://github.com/marshmallow-code/marshmallow/pull/1066#issuecomment-498268715

Start by reading Schema._normalize_nested_options and Nested._nested_normalized_option, then review the linked pull requests and their discussion. Trace how modifiers are accepted as Nested parameters, Meta parameters, at load time, in schema initialization, and as dot-delimited strings. Done means the modifier handling is harmonized and the two normalization methods are merged.

Written by the indexing model from the issue text.