Allow custom placeholders in formatCharacters

I propose an addition to the API where you could add a placeholder element to override
the default placeholder character via `formatCharacters`.

Current behaviour: all characters are applied the same placeholder character, no matter the pattern.

Proposed behaviour: all symbols get the same placeholder character _by default_ but it can be overwritten when passing in a pattern, like:
```
var mask = new InputMask({
pattern: '11ww', // An uppercase letter followed by 5 word characters
formatCharacters: {
'w': {
validate: function(char) { return /\w/.test(char) }
transform: function(char) { return char.toLowerCase() },
placeholder: 'A'
}
}
})

// results in
_ _ A A
```

Or a separate option after `formatCharacters` so we could also override the placeholders
for `0`, `A`, `a`?