Run Rubocop in Git's pre-commit hook

pre-commit

```
#!/bin/sh
echo "\nRunning rubocop...\n"

declare -a ERRORS=()

ERRORS=("$(rubocop $(git diff --cached --name-only | tr -s "\n" " ")  | grep -e 'C:' -e 'E:')")

if [[ "$ERRORS" != "" ]]; then
	echo "\n BEE-BOP! There are some things that you need to fix before commiting!\n"
	history -p "${ERRORS[@]}"
  exit 1
fi

echo "Done! Rubocop has no complains!\n"

exit 0
```

#####
Two steps to get it working:
1) Run this command in your root directory: `mv .git/hooks/pre-commit.sample .git/hooks/pre-commit && chmod +x .git/hooks/pre-commit`
2) Replace the content of the file with the bash script above.

NOTE:
- The first line tells it what intepreter to use, here it's using bash. You can do #!/bin/sh ruby if you want to write a Ruby script.
- `mv` command is to rename the `.githook` from a sample to a legit one. Then `chmod` makes it executable so that it'll actually run.