Added option to ignore files

Added an 'ignorePattern' option that gives the possibility to declare a pattern with which you can ignore files on uglifying.
Reason is mainly for skipping already minified files which usually are in the same folder as unminified files ( libraries like jQuery or knockout for example ).


```javascript
.pipe(uglify({
    ignorePattern: '**/*.min.js'
}))
```

This option will save time/speed cause we skip dealing with already minified files, and we don't have to write extra code to split/merge streams so we can include the minified/unminified files.

I don't know if it's needed to include 'minimatch' in the package.json.

Also I removed an assignment to ```options.outSourceMap``` which I don't really know how it's used.