Fix usage as a local dependency

[anri-asaturov]

With this changes diff2html-cli supports local dependency style of usage.
Example:
1 - install package locally

npm i -D diff2html-cli

2 - add a script to package.json:

scripts: {
  'show-diff': 'diff2html -- -M HEAD~1'
}

3 - then run in console

npm run show-diff

[rtfpessoa]

LGTM 👍