Add support for specifying the license file in the config file

[alex1989hu]

I would like to specify license header file. It is common to store a license file e.g. LICENSE.txt or LICENSE within the repository. The license file path option shall be mutually exclusive with the current option, header.

  - type: license
    spec:
      skipPaths:
        - .git/
        - .build*/
      includeSuffixes:
        - .ext
      excludeSuffixes:
        - .exclude-ext-prefix.ext
      headerPath:
        - LICENSE.txt

[andrewrynhard]

Sounds like a reasonable request. I wonder if we can do any trickery with the existing header field so that it can do both. Will think on it.