Add optional flag for ignoring vendor directory

[artuross]

Currently, gci formats files stored in vendor directory. With vendoring enabled in a repository, it requires gci to be executed for example with find which is not suitable:

find . -type f -name '*.go' -not -path "./vendor/*" -exec gci write --skip-generated --custom-order -s 'standard,default' {} \;

I suggest to add another feature flag, similar to --skip-generated to natively ignore vendor directories: --skip-vendor.

[daixiang0]

Fixed by #139