Shifting the existing input instead of rewriting -- possibly need an option

Hello !

Demo (video): http://take.ms/iJkM7

A brief description of the expectations below:

1. I fill in an [IBAN](https://en.wikipedia.org/wiki/International_Bank_Account_Number). It is being formatted like:
`1234 ABCD EFGH 0123`
2. Then I move the caret to some character in the middle of masked input (let it be right after `B` symbol ):
`1234 ABCD EFGH 0123`
`.......^ `
3. Then I press backspace and get
`1234 ACDE FGH0 123`
`......^ `
4. After that I try to type B once again and get
`123 ABDE FGH0 123` instead of naively) expected `1234 ABCD EFGH 0123`

The settings of the field are following:
```js
{
mask: '#### #### #### #### #### #### #### #### ##',
size: 42,
placeholderChar: ' ',
placeholderFixed: '#### #### #### #### #### #### #### #### ##',
hidePlaceholder: true,
isRevealingMask: true
},
```

P.S. Doing this without a placeholderFixed and a visible placeholder wont do, as IBAN is a format of arbitrary size, so when I enter something like `AD12 0001 2030 2003 5910 0100` and then try to change it to like `AT61 1904 3002 3457 3201` (which is also a valid IBAN) I will see `AT61 1904 3002 3457 3201 XXXX` where X is a placeholder char -- it won't disappear even if I clean all input.

Thanks in advance!