add two new flags for glob support

[ksdpmx]

There're some hardcoded directories and files excluded implicitly.
And sometimes we may need some kind of flexibility for specifing exclusions,

1. ignoring some files inside of one folder
2. ignoring multiple folders with naming convention.

So here introducing two new opt-in flags:

• -exclude-dirs
• -exclude-files

The default behavior for these two flags aligns with the existing -excludes for backward compatiblity.
And glob matching is supported, e.g. ./ltag -check -v -path ./test -exclude-dirs ".git .svn vendor image/module[abc]" -exclude-files "LICENSE MAINTAINERS OWNERS test/temp.go test/internal/foo*.go".
-excludes can be deleted if breaking change is acceptable.