feat: improve behavior when config file is missing

[calebcartwright]

Changes

• [X] Externalizes the hook script contents into a script file
• [X] Enhances the rusty-hook cli to respond with a specific exit code (currently 3) when the internal error was a missing config file
• [X] Enhances the git hook scripts to look at the exit code from rusty-hook.
  • Obviously on a zero exit code, everything will pass/proceed as normal
  • On a non-zero exit code, the script will print a message stating which git hook was rejected
  • On a missing config file exit code, the hook will instead exit with 0, allowing the git hook workflow to continue. The pre-commit hook will also display a warning message to the user:

    
    rusty-hook git hooks are configured, but no config file was found
    In order to use rusty-hook, your project must have a config file
    See https://github.com/swellaby/rusty-hook#configure for more information

If you were trying to remove rusty-hook, then you should also delete the git hook files to remove this warning

## Related Issues
<!-- List any related/impacted issues -->
- Closes #52 
- Closes #49

[codecov[bot]]

Codecov Report

Merging #53 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #53   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines         134    141    +7     
=====================================
+ Hits          134    141    +7

Impacted Files	Coverage Δ
src/config.rs	100% <100%> (ø)	:arrow_up:
src/git.rs	100% <100%> (ø)	:arrow_up:
src/rusty_hook.rs	100% <100%> (ø)	:arrow_up:

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[traviskosarek]

I agree, looks like a good addition