Add ELF support to CrashDebugCommandLine module

Can now specify --elf command line flag instead of --bin to have
CrashDebug load the FLASH contents from a .elf executable instead of a
.bin image.
CrashDebug issues more error text

Previously the user would just be notified of a file reading error
from within main() but now it issues messages to stderr for ELF
parsing errors and invalid command line arguments as well.
Update README to doc --elf command line option