Customize License check string

License could have different formats. For Instance, instead of string "copyright" some licenses just has "(c)". As of now this plugin just checks for string "copyright". 
https://github.com/google/addlicense/blob/a0294312aa76d31c0bd7e49083d88a2a04d9b3d1/main.go#L289
it would be nice to have customize checks.