make GetFlagString() process the default string

[KonradIT]

Original PR here: https://github.com/KonradIT/mmt/pull/127

make GetFlagString() process the default value, not Cobra

since the dateFormat / --date has a default set to dd-mm-yyy, the GetFlagString() function does not ever attempt reading the value from the config file.

Now every GetFlagString() invocation explicitly lists the default values, like GetFlagInt() does, hence one can set the date format in the config file.

Type:

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Documentation

Camera:

• [x] GoPro
• [x] Insta360
• [x] DJI
• [x] Android
• [ ] This PR adds a new camera

Component:

• [x] Core logic
• [ ] Import
• [ ] Merging
• [ ] Firmware Update
• [ ] Video Manipulation

Checklist before approval:

• Linter pass
• Build pass