fully implement shrink command

[plastikfan]

builds upon #3 to complete the implementation

• implement look-ahead #53
• sample feature #48
• inline mode #49
• full run #50
• journalling #51
• extension mapping #52
• add suffixes into advanced config #107
• validate user specified file suffixes #110
• rework config #112
• tidy up #114
• implement dry run
• implement timeout
• make magick real (to be implemented under this issue; remove the dummy and make effective)