Customise cache and downloader

[onmyway133]

• This tries to fix https://github.com/hyperoslo/Imaginary/issues/74
• Allow customising Storage
• Allow customising ImageDownloader together with a modify request closure

We get some logs

2018-01-02 12:53:31.080357+0100 ImaginaryDemo[90905:733957] Task <A4C7C3D2-0730-4E37-93D5-8FC7A93DC219>.<3> finished with error - code: -999
2018-01-02 12:53:31.130806+0100 ImaginaryDemo[90905:733943] Task <4BCA1ED2-223E-489D-94A6-E0894D0B3D4C>.<1> finished with error - code: -999

This is because of excessive logging in iOS 11 https://forums.developer.apple.com/thread/88336