Add support for inlining CSS stylesheets. Add tests. Update docs.

lib/link-css.js

@@ -0,0 +1,70 @@
+const co = require('co');
+const fs = require('mz/fs');
+const isLocalPath = require('is-local-path');
+const isTemplateExpression = require('./is-template-expression');
+const path = require('path');
+const postcss = require('postcss');
+const postcssImport = require('postcss-import');
+const postcssUrl = require('postcss-url');
+const R = require('ramda');
+
+const forEachIndexed = R.addIndex(R.forEach);
+const render = R.curryN(2, co.wrap(function * (filename, href) {
+  var imports;
+  const processor = postcss()
+    .use(postcssImport({ async: true, onImport: files => imports = files }))
+    .use(postcssUrl({ url: 'rebase' }));
+  try {
+    const css = yield fs.readFile(href, 'utf8');
+    const result = yield processor.process(css, { from: href, to: filename });
+    const output = {css: result.css, imports};
+    return output;
+  }
+  catch (error) {
+    // process uses error.file = href
+    // import uses error.fileName
+    // readFile uses nothing => use filename
+    error.filename = error.file || error.fileName || filename;
+    throw error;
+  }
+}));
+/**
+ * Inline liked CSS stylesheets by replacing link elements with
+ * style elements that contain the css file contents.
+ * @param {Object} $
+ * 	Parsed HTML source to inline
+ * @param {String} [filename='.']
+ *  Filename of the HTML document contained within $
+ * @return {Promise}
+ */
+const inlineLinkCss = co.wrap(function * ($, filename) {
+  // TODO consider: explicitly default filename = '.'?
+  // path.dirname(null || undefined) -> '.'
+  const basedir = path.dirname(filename);
+  var files = [];
+  try {
+    const links = $('link[rel="stylesheet"]')
+			.filter((index, link) => {
+				const href = $(link).attr('href');
+				return isLocalPath(href) && !isTemplateExpression(href);
+			})
+			.toArray();
+    const getHref = element => path.resolve(basedir, $(element).attr('href'));
+    const hrefs = R.map(getHref, links);
+    files = R.concat(files, hrefs);
+    const outputs = yield R.map(render(filename), hrefs);
+		const imports = R.flatten(R.map(R.prop('imports'), outputs));
+		files = R.concat(files, imports);
+    const styles = R.map(output => $('<style>').html(output.css), outputs);
+		const replaceLink = (link, index) => $(link).replaceWith(styles[index]);
+		forEachIndexed(replaceLink, links);
+		files = R.uniq(files);
+		return { $, files };
+  }
+  catch (error) {
+    if (!error.filename) error.filename = filename;
+		error.files = R.uniq(files);
+		throw error;
+  }
+});
+module.exports = inlineLinkCss;