update: Improve Cat utility with options, docs, and tests

[NitkarshChourasia]

Summary

Improves the Cat/cat.py program by making it closer to the Unix cat command and easier to test/use.

Changes

• Reads files one at a time instead of loading all files into memory
• Supports standard input when no file is provided
• Supports - for reading from standard input between files
• Prints file errors to stderr and continues processing remaining files
• Adds proper exit codes
• Adds command-line options:
  • -n / --number
  • -b / --number-nonblank
  • -s / --squeeze-blank
  • -E / --show-ends
• Adds README.md with usage examples
• Adds unittest test cases for the Cat program

Testing

Ran:

python -m unittest test_cat.py