Add exclude pattern for git checks

[dmcgowan]

Allows excluding of vendor directories for whitespace checks

see failure for containerd/containerd#771

$ TRAVIS_COMMIT_RANGE="${TRAVIS_COMMIT_RANGE/.../..}" make dco
git-validation -q -run DCO,short-subject,dangling-whitespace
 fcef902 - FAIL - has whitespace errors. See `git show --check fcef902aa5583e3efde5456562fd84e09c835258`.
1 commits to fix
make: *** [dco] Error 1

The problem was in a vendored README

vendor/github.com/godbus/dbus/README.markdown:28: trailing whitespace.
+gives a short overview over the basic usage. 

This allows setting GIT_CHECK_EXCLUDE="./vendor" for go projects to exclude the check here

[vbatts]

tough problem. thanks for this. I'll add docs in a separate pr
LGTM