Use OptionGroup for hack

- Close issue #26 (user can now use cdiff -s -U5 --cached)

Use `--no-ext-diff` to disable GIT_EXTERNAL_DIFF and diff.external which might
break cdiff output

Temporarily unset GIT_EXTERNAL_DIFF when invoking git diff.

When the git diff command is customized through setting GIT_EXTERNAL_DIFF in the environment, the output of cdiff is broken and defaults to the output of the tool git has been configured to use.

Temporarily unsetting this variable when cdiff invokes git diff fixes the issue.

- Pypi requires strict reStructuredText syntax
- New make target `doc-check`

If need to support normal diff, writing a ndiff to udiff translator
makes more sense