Add txt import so file can be imported/exported from symmetrically

src/lib/import-csv.js

@@ -3,7 +3,7 @@ import Papa from 'papaparse';
 export default () => new Promise((resolve, reject) => {
     const fileInput = document.createElement('input');
     fileInput.setAttribute('type', 'file');
-    fileInput.setAttribute('accept', '.csv, .tsv'); // parser auto-detects delimiter
+    fileInput.setAttribute('accept', '.csv, .tsv, .txt'); // parser auto-detects delimiter
     fileInput.onchange = e => {
         const file = e.target.files[0];
         Papa.parse(file, {