Better config setting for custom columns

Wrap-guide currently has one of the worst config setting layouts of any package.

It requires an array of objects to configure custom columns and each object in the array has to have  a `pattern` and `column` field.

I'm proposing greatly simplifying this by making `wrap-guide.columns` an object where the keys are extensions and the values are column numbers.

### Current

```coffee
'wrap-guide':
  'columns': [
    {
       pattern: '\\.mm$'
       column: 120
    }
    {
      pattern: '.java$'
      column: 100
    }
  ]
```

### Proposed

```coffee
'wrap-guide':
  'columns':
    'mm': 120
    'java': 100
```