Add mattermost adapter (#49) (#50)

lib/notification/adapter/mattermost.js

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+const { markdown2Html } = require('../../services/markdown');
+const { getJob } = require('../../services/storage/jobStorage');
+const axios = require('axios');
+
+/**
+ * sends new listings to mattermost
+ * @param serviceName e.g immowelt
+ * @param newListings an array with newly found listings
+ * @param notificationConfig config of this notification adapter
+ * @param jobKey name of the current job that is being executed
+ * @returns {Promise<Void> | void}
+ */
+exports.send = ({ serviceName, newListings, notificationConfig, jobKey }) => {
+  const { webhook, channel } = notificationConfig.find((adapter) => adapter.id === 'mattermost').fields;
+  const job = getJob(jobKey);
+  const jobName = job == null ? jobKey : job.name;
+
+  let message = `### *${jobName}* (${serviceName}) found **${newListings.length}** new listings:\n\n`;
+  message += `| Title | Address | Size | Price |\n|:----|:----|:----|:----|\n`;
+  message += newListings.map(
+    (o) => `| [${o.title}](${o.link}) | ` + [o.address, o.size.replace(/2m/g, '$m^2$'), o.price].join(' | ') + ' |\n'
+  );
+
+  return axios.post(`${webhook}`, {
+    channel: channel,
+    text: message,
+  });
+};
+
+/**
+ * exported config is being used in the frontend to generate the fields
+ * incoming values will be the keys (and values) of the fields
+ *
+ */
+exports.config = {
+  id: __filename.slice(__dirname.length + 1, -3),
+  name: 'Mattermost',
+  readme: markdown2Html('lib/notification/adapter/mattermost.md'),
+  description: 'Fredy will send new listings to your mattermost team chat.',
+  fields: {
+    webhook: {
+      type: 'text',
+      label: 'Webhook-URL',
+      description: 'The incoming webhook url',
+    },
+    channel: {
+      type: 'text',
+      label: 'Channel',
+      description: 'The channel where fredy should send notifications to.',
+    },
+  },
+};

lib/notification/adapter/mattermost.md

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+### Mattermost Adapter
+
+For Mattermost, you need to create a incoming webhook. This is pretty easy. Please visit the steps in the  [developer docs](https://docs.mattermost.com/developer/webhooks-incoming.html) and follow the instructions.
+
+As a result, you get the webhook URL for configuration in fredy. In addition, the target channel must be defined.