Multiplefileinput

#### Main purpose of commit:
Ability to minify multiple files without concatenating them:
```javascript
options: {
  'concat': false
},
files: {
  'dest/': 'src/*.html'
}
```


#### Changes:
* Added **concat** option. Defaults `true`, so plugin is downwards-compatible.
* Logic to minify multiple files at same loop
* Renamed all **multiple** tests to **concat** tests
* Added new **multiple** tests to test output with multiple files without concatenation


#### Logic of the multiple file loop:
All files are sent to the `files.forEach()` loop.
Here's the difference between concat option, with file array of `['xx','xx']`

Concat value: `true`
Variable sent to the forEach will be `[['xx','xx']]`, so they will be concatenated.

Concat value: `false`
Variable sent will be `[['xx'],['xx']]`, so the files will be parsed one-by-one.