Customize License check string

[Sanjeev Kumar]

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".

addlicense/main.go

Line 289 in a029431

 return bytes.Contains(bytes.ToLower(b[:n]), []byte("copyright")) || 

it would be nice to have customize checks.