Allow mixing project and dlt file arguments directly

[tohammer]

Decide which one it is based on extension. Simplifies usage from command line when dlt and project files are grouped in directories.

[alexmucde]

@tohammer Please can you also here update the option documentation? I think the options -l and -p are not needed anymore, DLT files (incl. multiple files) and project files can be now loaded without options.

[tohammer]

@alexmucde Should i remove the -l and -p options completely or keep them in code for compatibility? (and ofc will update release info/docsaccordingly)

[alexmucde]

@tohammer You can remove them completely

[alexmucde]

@tohammer I will merge now you commit and do the requested changes by my own, because i need further modifications.