Pull license value from license file

[jamiemjennings]

npm allows for the license to be specified in an external file: https://docs.npmjs.com/files/package.json#license

Example (package.json):

{ "license" : "SEE LICENSE IN <filename>" }

Currently npm-license-crawler will return:

├─ foo-module@1.2.3
│  ├─ licenses: SEE LICENSE IN LICENSE.txt

It would be preferable for it to return the contents of LICENSE.txt as the licenses output.

[mwittig]

Dumping the file contents to the licenses output may break the output.

I need to check to what extent --relativeLicensePath covers the "SEE LICENSE IN" case. It should work for at least if the filename is LICENSE or LICENSE.txt