Add regex based grouping logic

[mwestphal]

Is your feature request related to a problem? Please describe.
With the addition of a grouping logic (--multi-file-mode) in #1609, it would be nice to have more grouping logic possibilities.

Describe the solution you'd like
We currently support all and single, but a new regex mode could be added, associated with another option, --multi-file-regex.

Here is how it could work:

Regex: (.*.obj)|(.*.gltf)
Files:

file1.gltf
file2.gltf
file.obj
file2.obj

Results:
Two group of files:
Group1:

file1.gltf
file2.gltf

Group2:

file.obj
file2.obj

The command line would look like this:

./bin/f3d file1.gltf file2.gltf file.obj file2.obj --multi-file-mode=regex --multi-file-regex=(.*.obj)|(.*.gltf)