[MISC]: Amended TODO
[FEATURE]: Added shell_error() to shell.c and corresponding macros to shell.h.
[FEATURE]: Replaced multiple non-standard error reporting with shell_error()
[BUGFIX]: Added -DDEBUG flag to debug build flags in Makefile. Added #ifdef DEBUG to report_error calls
[BUGFIX]: Added shell_error() to error handling code in case fork() fails in execute_command(). Added corresponding ERR_SHELL_ERROR macro and error string in shell_error() array. Added in forgotten error string in shell_error() array.
[BUGFIX]: Replaced malloc/strncpy with copy_string() in count_token()
[MISC]: Amended .gitignore
[FEATURE]: Added proper signal handling for both shell and spawned processes
[MISC]: Added comments to both shell.h and env.h
[BUGFIX]: Added wit to otherwise un-witty error string
[MISC]: Forgot to amend TODO file
[FEATURE]: Added ability to set system environment variables from builtin shell variables. Added to_lower_varname()/to_upper_varname() and set_sys_env_var() to accomplish this. Also added _POSIX_C_SOURCE feature test macro to Makefile to access setenv while using -std=c11.
[CLEANUP]: Added silly-missing semicolon