Rename Folders with Year Month and the *Day* of the first (youngest) picture taken: `%firstday` placeholder

[rafo]

That sounds esoteric, but allow me to explain:

I did numerous trips in one month and trips that span more than one month. To have the folders sorted by time it would be great to add the day of the youngest picture in the folder.

Instead of:

├── 201506_Berlin_Germany
│   ├── 20150629_16-34-14-img_3900.jpg
│   └── 20150630_17-07-06-img_3901.jpg
├── 201506_Gradac_Croatia
│   ├── 20150619_17-16-37-img_9426.jpg
│   └── 20150624_19-06-33-img_9432.jpg
├── 201506_Vienna
│    ├── 20150612_01-41-38-_dsc8705.dng
│    ├── 20150612_01-41-38-_dsc8705.nef
├── 201507_Berlin_Germany
    ├── 20150701_16-34-14-img_3900.jpg
    └── 20150701_17-07-06-img_3901.jpg

Berlin was one destination or let's say one project, but since it goes from June to July its in two different folders, with two to n different folders in between.

It should be like:

├── 20150612_Vienna
│    ├── 20150612_01-41-38-_dsc8705.dng
│    ├── 20150612_01-41-38-_dsc8705.nef
├── 20150619_Gradac_Croatia
│   ├── 20150619_17-16-37-img_9426.jpg
│   └── 20150624_19-06-33-img_9432.jpg
└── 20150629_Berlin_Germany
    ├── 20150629_16-34-14-img_3900.jpg
    ├── 20150630_17-07-06-img_3901.jpg
    ├── 20150701_16-34-14-img_3900.jpg
    └── 20150701_17-07-06-img_3901.jpg

Because Berlin was the last destination, starting end of June but going into July, It should be at the end of the list. This could be easyely accomplished with a %firstday placeholder (or however you want to name it).