SwiftLint2

Update yaml parsing to allow single value configurations

This updates the `arrayOfStrings` and `arrayOfInts` methods to also
accept a single value if they are not parsed as an array. This lets us
do this for our configuration:

    line_length: 110

Instead of:

    line_length:
      - 110

If we only actually need a single value. The same goes for source
directories and other string keys.