handles nil values encoded as empty strings (#49)

Adds basic combine support (#45)

* adds basic combine support
* make coherent with TopLevelDecoder
* don't decode all elements if we only need one

Fix sample app: nested paths during decoding, supporting optional val…

…ues (#28)

* use nested codingPath in unbox<T>(_:)
* add test for nested types
* use nestedPath symbol name like elsewhere
* add return for consistency
* fix optionalPig test
* disambiguate "price" when cell not found
* fix optional empty list tests
* add TODO for potential other test
* add test for optional columns without values
* add table headers for nil properties
* remove empty extension
* encode table headers for empty string cells

Fixed #1
* test multiple optional empty string values
* implement encoding and skipping optional values in optional subtypes

[Don't merge] Semver + header (#32)

* semver + header
* bump swift-markdown

bump swift-markdown

semver + header