declare a source file even when passing in a string

when generating source maps by using the <code>fromString=true</code> in the <code>Uglify.minify</code> options, the source map contains a problematic array for the sources field

```javascript
...
sources: ["?"],
...
```

thus, the source map can't be used because i can't point it at the original source. however, if i do not use a string directly but the filename itself, the source map feature works.

this pull request adds a <code>fromFile</code> option that can either be an array of filenames or just a single string filename to properly match up if just a single string of source code is passed in, or an array of source code.

so the following approaches now produce good source maps

```javascript
Uglifyjs.minify(
  "function() { alert('yes');", 
  {fromString:true, fromFile:"yes.js"});
```

and

```javascript
Uglifyjs.minify(
  ["function() { alert('yes');", "function() { alert('no');"], 
  {fromString:true, fromFile:["yes.js", "no.js"]});
```