License header URL support

[bpayne55]

A group can have multiple projects with the same license header. Is it possible to support a link to the header text? This way, the license header doesn't need to be checked into every repository. It can be hosted in one place.

I believe the place the change would happen would be here, which might be fairly straightforward.

spotless/plugin-maven/src/main/java/com/diffplug/spotless/maven/generic/LicenseHeader.java

Lines 63 to 70 in bb27596

	private String readFileOrContent(FormatterStepConfig config) throws IOException {
	if (content != null) {
	return content;
	} else {
	byte[] raw = Files.readAllBytes(config.getFileLocator().locateFile(file).toPath());
	return new String(raw, config.getEncoding());
	}
	}

[nedtwigg]

I strongly recommend blowdryer for this. There are lots of places where Spotless (and other plugins) has a file as an input, and people want to pass a URL, here's another example.

We aren't going to merge support for URLs directly into Spotless, because that is best handled at a different level.