This adds support for serveral new command line options (taken from the usage):
    - `-b, --bin-dir   Override the bin installation directory`
    - `-m, --man-dir   Override the man installation directory`
    - `-a, --arch      Override the architecture identified by the installer`
    - `-h, --help      Display this help message`
This also (coincidentally) allows for $BIN_DIR, $MAN_DIR, and $ARCH to be set to create default settings for these.
These variables are *always* overwritten by the command line arguments.

This helps ensure that mismatched permission on `_bin_dir` and `_man_dir` don't cause issues.
Also prevents creating man pages owned by root being written to the user's $HOME

This prevents attempting to create files in `/` if a path is not given, instead failing early.

They now suggest passing to `sh`, rather than `bash`, to match the shebang and reduce posibility that non-posix behaivor of `bash` affects the script.

this was requested by @ajeetdsouza

Since the installation instructions now recommend using `sh`, check that this implementation *does* support local before continuing.

This ensures that odd values for _bin_name, _bin_dir, and others aren't treated as options to the command and cause a failure.

Remove the display of this default value, since we can't detect it without erroring before usage.

the ci requires that you run `shfmt -w --indent=4 --language-dialect=posix --simplify *.sh'`, and as such, i now have.

note: I also formatted the case statement in parse_args to fit on one screen while still being readable.