`ignore` parameter

I think it could be really cool to add an optional parameter to ignore some words.

Example:

```python
>>> parse('twenty one')
'21'

>>> parse('twenty one', ignore=["one"])
'20 one'
```
or

```python
>>> parse('I have three apples and one pear.')
'I have 3 apples and 1 pear.'

>>> parse('I have three apples and one pear.', ignore=["three"])
'I have three apples and 1 pear.'
```


