Add split_Command function and complete handle_Command function
Make internal command "echo" completely
Make --help, --version option and export command
Resolve getopt bug.
Reset optind variable to 0.
Make new files for built-in functions and add help for export command
Move files to builtin directory
Add ls command
Add printing environment variables function
Add man page of echo
Add man page of export, ls and whoami
Add cd, pwd and &(run in background) functions
Merge branch 'develop'