Allow fixed-size versions to be aligned, allow many fixed-size versions for single target

- Many fixed size copies may be generated for single target (by passing an array instead of object)
- fixed size copies may be aligned horizontally and vertically (for example top-left)

No breaking changes are introduced.

In next minor release (0.5.0) I suggest:
- replacing `fixedSizeVersion` with `fixedSizeVersions` for consistency and dropping support for both `true` and object values for this setting: only array input should be supported.
- dropping support for default suffix of `-fixed-size` in favour of `-center-middle`