added file.progress property, along with documentation

docs/api.md

@@ -469,6 +469,10 @@ File length (in bytes), as specified by the torrent. *Example: 12345*
 
 Total *verified* bytes received from peers, for this file.
 
+## `file.progress`
+
+File download progress, from 0 to 1.
+
 ## `file.select()`
 
 Selects the file to be downloaded, but at a lower priority than files with streams.

lib/file.js

@@ -56,6 +56,10 @@ Object.defineProperty(File.prototype, 'downloaded', {
   }
 })
 
+Object.defineProperty(File.prototype, 'progress', {
+  get: function () { return this.length ? this.downloaded / this.length : 0 }
+})
+
 File.prototype.select = function (priority) {
   if (this.length === 0) return
   this._torrent.select(this._startPiece, this._endPiece, priority)