Fix flycheck issues

Fix issues with `flycheck` checker. 

- Converted `flycheck-swift-sdk-path` to the appropriate type;
- Fixed issues with `nil` value for `flycheck-swift-linked-sources`;
- Update `source` extraction from the `flycheck-substitute-argument`;
- Disabled checker by default. It doesn't work well with real word projects, since it doesn't support linked libraries.

fixes #54