add prefix match exclude feature

[buhuipao]

Add prefix match exclude feature, such as

$ ./gocmt -d testdata/pkg -e 'testdata/pkg/xxx/'
ignore testdata/pkg/xxx/xxx.go
testdata/pkg/yyy/yyy.go no changes

or

$ ./gocmt -d testdata/pkg -e 'testdata/pkg/xxx/,testdata/pkg/yyy/'
ignore testdata/pkg/xxx/xxx.go
ignore testdata/pkg/yyy/yyy.go

[alswl]

Vote for this pull requests.

When you must dealing with many zz_generated_xxx, the exclusion features will helps you a lot.