add tweet api function

lib/api.js

@@ -1,5 +1,7 @@
 import axios from 'axios'
 
+const DOMAIN = process.browser ? document.location.origin : ''
+
 const gistClient = axios.create({
   baseURL: 'https://api.github.com',
   timeout: 5000,
@@ -9,6 +11,16 @@ const gistClient = axios.create({
   }
 });
 
+async function tweet (encodedImage) {
+  const processedData = encodedImage.split(',')[1]
+
+  return axios.post(`${DOMAIN}/twitter`, { data: processedData })
+  .then((res) => res.data.url)
+  .then((url) => encodeURIComponent(`Built with @carbon ${url}`))
+  .then((uri) => `https://twitter.com/intent/tweet?text=${uri}`)
+  .then(openTwitterUrl)
+}
+
 async function uploadImage (encodedImage) {
    // upload image
   const url = 'https://api.imgur.com/3/image'
@@ -40,5 +52,16 @@ const getGist = (id) => {
 
 export default {
   uploadImage,
-  getGist
+  getGist,
+  tweet
+}
+
+// private
+function openTwitterUrl (twitterUrl) {
+  const width  = 575, height = 400
+  const left = (window.outerWidth  - width)  / 2
+  const right = (window.outerHeight - height) / 2
+  const opts = `status=1,width=${width},height=${height},top=${top},left=${left}`
+  
+  window.open(twitterUrl, 'twitter', opts)
 }