Support watching for source changes

Plugins have to call `addWatchFile` now.
https://rollupjs.org/migration/#changes-to-the-plugin-api
https://rollupjs.org/plugin-development/#this-addwatchfile

Attempts to fix 2wce#58.

src/index.ts

@@ -1,5 +1,6 @@
 import pluginUtils from '@rollup/pluginutils';
 import fs from 'fs';
+import { resolve as resolvePath } from 'path';
 import { ExistingRawSourceMap, Plugin, PluginContext } from 'rollup';
 import { promisify } from 'util';
 import { resolveSourceMap, resolveSources } from './source-map-resolve';
@@ -27,6 +28,9 @@ export default function sourcemaps(
         return null;
       }
 
+      // Allow the original file to be watched
+      this.addWatchFile(resolvePath(id));
+
       try {
         // Try to read the file with the given id
         code = (await promisifiedReadFile(id)).toString();