Converted some strings to f-strings.
Fixed some grammar in error messages and docs.
Increased code coverage.

Changed CommandResult.stderr to a str instead of Optional[str]

# Conflicts:
#	cmd2/__init__.py
#	cmd2/argparse_completer.py
#	cmd2/argparse_custom.py
#	cmd2/cmd2.py
#	cmd2/decorators.py
#	cmd2/exceptions.py
#	cmd2/utils.py
#	examples/arg_decorators.py
#	examples/argparse_completion.py
#	examples/modular_commands_main.py
#	tests/test_argparse_completer.py
#	tests/test_argparse_custom.py
#	tests/test_cmd2.py
#	tests/test_completion.py
#	tests/test_history.py