#596 Allow alternative file extensions

[Benedikt Freisen]

There is currently no way to specify alternative file extensions for sdcc's input and output files.
It it therefore not easily possible to compile source files with an extension other than ".c".
An ability to choose between different extensions for output files would be equally useful (e.g. ".asm" or ".S", ".rel" or ".o" etc.).

Both could be done via additional command line options, e.g. --src-file-ext, --obj-file-ext and --asm-file-ext with the current behavior as default setting.

[Benedikt Freisen]

This is a first patch.

It adds the command line options --src-ext, --asm-ext, --obj-ext and --lib-ext to the compiler and --obj-ext, --lst-ext and --sym-ext to the assembler.

Let me know what you think of the approach.