add make targets for js,css, add javascript linter

- add `make js`, deprecating `make javascripts`
- add `make css`, deprecating `make generate-stylesheets` and
  `make stylesheets-check`
- changed the unclean css check to only run on CI
- add JS linting via eslint with basic configuration and fixed
  discovered issues
- changed autoprefixer to use official `postcss-cli` avoiding the need
  to loop in the makefile
- moved browserslist to package.json so other future tools can use it
  too.
- update documentation for new make targets and added JS section

public/js/draw.js

@@ -1,13 +1,15 @@
+/* globals gitGraph */
+
 $(document).ready(function () {
 	var graphList = [];
-	
+
 	if (!document.getElementById('graph-canvas')) {
 		return;
 	}
-	
+
 	$("#graph-raw-list li span.node-relation").each(function () {
 		graphList.push($(this).text());
 	})
-	
+
 	gitGraph(document.getElementById('graph-canvas'), graphList);
 })