Add Pushover notification adapter (#91)

* Add Pushover notification adapter

lib/notification/adapter/pushover.js

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+import { markdown2Html } from '../../services/markdown.js';
+import { getJob } from '../../services/storage/jobStorage.js';
+import fetch from 'node-fetch';
+
+export const send = ({ serviceName, newListings, notificationConfig, jobKey }) => {
+  const { token, user, device } = notificationConfig.find((adapter) => adapter.id === config.id).fields;
+  const job = getJob(jobKey);
+  const jobName = job == null ? jobKey : job.name;
+  const promises = newListings.map((newListing) => {
+    const title = `${serviceName}: ${newListing.title}`;
+    const message = `Address: ${newListing.address}\nSize: ${newListing.size}\nPrice: ${newListing.price}\nLink: ${newListing.link}`;
+    return fetch('https://api.pushover.net/1/messages.json', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        token: token,
+        user: user,
+        message: message,
+        device: device,
+        title: newListing.title,
+      }),
+    });
+  });
+
+  return Promise.all(promises);
+};
+
+export const config = {
+  id: 'pushover',
+  name: 'Pushover',
+  readme: markdown2Html('lib/notification/adapter/pushover.md'),
+  description: 'Fredy will send new listings to your mobile using Pushover.',
+  fields: {
+    token: {
+      type: 'text',
+      label: 'API token',
+      description: 'Your application\'s API token.',
+    },
+    user: {
+      type: 'text',
+      label: 'User key',
+      description: 'Your user/group key.',
+    },
+    device: {
+      type: 'text',
+      label: 'Device name',
+      description: 'The device name to send your notification to. Messages may be addressed to multiple specific devices by joining them with a comma.',
+    },
+  },
+};

lib/notification/adapter/pushover.md

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+### Pushover Adapter
+
+Refer to the [instructions](https://support.pushover.net/i7-what-is-pushover-and-how-do-i-use-it) to set up your Pushover application.
+
+After setting up the application, please enter both your newly created User key and API token.