Merge pull request JohnSundell#16 from JohnSundell/item-modified-date

FileSystem.Item: Add modifiedDate property

Merge pull request JohnSundell#11 from yageek/feature/currentFolder

Add syntactic sugar to reach the current folder.

Add APIs for checking whether a folder contains a given file or folder

It’s always been possible to check if a file exists using try?, but that
looks quite ugly, so let’s hide that ugliness behind a nice looking API :)

Merge pull request JohnSundell#8 from pixyzehn/fix-create-file-at-pat…

…h-method

Fix create file at path method

Support referencing a parent in a path using “../“

Files now correctly handles paths that contains parent references, by expanding
those references into absolute paths.

So ‘../file’ is expanded to ‘<currentFolderParent>/file’.

Replace custom `fileURL` property with `URL(fileURLWithPath:)`

Add @discardableResult to …ifNeeded methods

Enable using tilde in path when creating folder from FileSystem

You can now specify the user’s home directory using a tilde (~), when
creating a folder through `FileSystem.createFolder(at:)`.

Add Folder.createFileIfNeeded()

Just like `Folder.createSubfolderIfNeeded()`, this new API enables the user
to only create a file if one is missing.

Add Folder.subfolder(atPath:)

This enables easier access to a subfolder located deep within a folder’s
tree, instead of having to retrieve all the intermediate folders.